When I need some quick shims for off contact on one of my manual screen printing presses I keep on hand the following: popsicle sticks, index cards, vertical blind slats, and a cut up old mousepad.
Each have their uses for one job or another. Depending on the thickness of the off contact that I need, I grab what I need to make the job easier. I tape the shim to the bottom or the side of the screen depending on what works best.
The idea for me is to get through the job and get the shirts out the door.
Hope this tip can help you.
Gosset Screen Printing