There are lots of thumbwheel designs on Thingiverse. Many of them are very nice but lack a spacer that makes it easier to grab and rotate the thumbwheel. Rather than remixing another designer's thumbwheel, I took a more universal route. I made spacers.
All of them are 12mm maximum in diameter and have a hole for an M3 screw. No drilling required. Each spacer has a different height. The STL files are labeled as such.
Helpful Tip: Since these are so small and fast to print, I recommend you print four of each size all at once. That way you will have the right size on hand while your print bed is out of commission.
And while you are redoing your printer bed, if you have an Anet A8, you may want to check out my other things to help you perfectly square and level your print bed to your printer's frame: https://www.thingiverse.com/thing:3087459