I've got an old filter holder that I should replace the foam on. I've seen people recommend mouse pads. Considering I don't need much what would be a good replacement? I'm worried about damaging the filters.

You can buy replacement foam in various widths and thicknesses from www.micro-tools.com.

Thanks. Do you think it really matters which of the foams they list I choose? One of the light baffles that come in 10" squares sounds best. Just cut it to size and cut out the hole.

Hard to say without seeing the thing it's being applied to.

It's a Kenko Technical holder. Basically the two halves are held together by a little screw. It wouldn't seem to be too critical when it comes to thickness. If the foam wasn't so thick that the whole thing couldn't close I can almost believe anything would work.