A simple 20x20mm square slat, at 45 degrees, with a simple notch to know orientation.
You can quickly print them to check and fine-tune your belts at your CoreXY system.
In order to do that:
1- Set your X and Y steps/mm to theoretical values.
2- Print this thing and measure both sides. There is no problem if they are not 20mm but they must be equal.
3- If they are not equal then:
If A>B then tight the A or lose the B belt
If B>A then tight the B or lose the A belt
When they are equal then you are set to calibrate your steps/mm for accurate printing. I strongly recommend this thing:
You can also print 4 of these slats, one each corner, to check if your printer has parallelism issues.