Instead of putting something on your monitor, just use AHK.

#SingleInstance force
#Persistent
OnExit, ExitRoutine
xpos := A_ScreenWidth/2
ypos := A_ScreenHeight/2
hDrwArea := DllCall("GetDC", "uint", Null)
SetTimer, SetPixel, 1
SetPixel:
DllCall("SetPixel", "uint", hDrwArea, "int", xpos, "int", ypos, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos, "int", ypos - 2, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos, "int", ypos + 2, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos - 2, "int", ypos, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos + 2, "int", ypos, "uint", 255, "uint", 0, "uint", 0)
; ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
DllCall("SetPixel", "uint", hDrwArea, "int", xpos - 6, "int", ypos - 6, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos - 6, "int", ypos - 6, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos + 6, "int", ypos + 6, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos + 6, "int", ypos + 6, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos - 6, "int", ypos + 6, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos + 6, "int", ypos - 6, "uint", 255, "uint", 0, "uint", 0)
; ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
DllCall("SetPixel", "uint", hDrwArea, "int", xpos - 9, "int", ypos - 9, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos - 9, "int", ypos - 9, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos + 9, "int", ypos + 9, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos + 9, "int", ypos + 9, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos - 9, "int", ypos + 9, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos + 9, "int", ypos - 9, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos, "int", ypos, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos, "int", ypos - 2, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos, "int", ypos + 2, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos - 2, "int", ypos, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos + 2, "int", ypos, "uint", 255, "uint", 0, "uint", 0)
; ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
DllCall("SetPixel", "uint", hDrwArea, "int", xpos - 6, "int", ypos - 6, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos - 6, "int", ypos - 6, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos + 6, "int", ypos + 6, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos + 6, "int", ypos + 6, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos - 6, "int", ypos + 6, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos + 6, "int", ypos - 6, "uint", 255, "uint", 0, "uint", 0)
; ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
DllCall("SetPixel", "uint", hDrwArea, "int", xpos - 9, "int", ypos - 9, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos - 9, "int", ypos - 9, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos + 9, "int", ypos + 9, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos + 9, "int", ypos + 9, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos - 9, "int", ypos + 9, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos + 9, "int", ypos - 9, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos, "int", ypos, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos, "int", ypos - 2, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos, "int", ypos + 2, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos - 2, "int", ypos, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos + 2, "int", ypos, "uint", 255, "uint", 0, "uint", 0)
; ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
DllCall("SetPixel", "uint", hDrwArea, "int", xpos - 6, "int", ypos - 6, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos - 6, "int", ypos - 6, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos + 6, "int", ypos + 6, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos + 6, "int", ypos + 6, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos - 6, "int", ypos + 6, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos + 6, "int", ypos - 6, "uint", 255, "uint", 0, "uint", 0)
; ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
DllCall("SetPixel", "uint", hDrwArea, "int", xpos - 9, "int", ypos - 9, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos - 9, "int", ypos - 9, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos + 9, "int", ypos + 9, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos + 9, "int", ypos + 9, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos - 9, "int", ypos + 9, "uint", 255, "uint", 0, "uint", 0)
DllCall("SetPixel", "uint", hDrwArea, "int", xpos + 9, "int", ypos - 9, "uint", 255, "uint", 0, "uint", 0)
Return
ExitRoutine:
DllCall("ReleaseDC", UInt, 0, UInt, hDrwArea)
DllCall("FreeLibrary", "UInt", hDrwArea)
ExitApp

Non-intrusive, though the flicker might annoy you. I personally sometimes forget to run it and only notice it after playing for awhile.