https://www.fpi-protostar.com/hitack.htm
This is the best stuff to use. Thin. No threads or specks either sticking up or dropping around. I used this on YashicaMats all the time to stop the internal reflections. Very effective.
Also good for thin light seals, but it isn't very compressible like foam.A 24 inch by 15 inch piece will last you a lifetime. Might even drop the company a note and ask if they can send you a few random cutoffs for a few dollars.
I occasionally buy a 24x30 inch sheet, chop it up, and as questions like yours come up, I'll throw some in an envelope and mail it out. Ask for a few dollars in return, which some times happens and sometimes doesn't. No biggie. But I am low and don't need to get a new piece. Maybe buy a chunk and run an ad in the sale section, keep bumping it up, you'll get rid of the extra over a few months.