I use the close cell neoprene that you can buy in rolls from an electronics store. It's made by 3M and comes in a variety of widths and thicknesses.
It is stickyback on one side only.

If needed I sometimes cut it down before or after installation with a sharp single edge razor blade.

Some of the non-neoprene foam does not hold up and deteriorates and crumbles with time.

Also, don't use the "car molding tape" found at auto parts stores, the adhesive is too strong and will not come off when needed.

DP

My box was leaking air on the bass side, some of the foam had dissapeared.

I bought a sheet of foam from the arts & crafts store after failed attempts of finding anything @the the hardware store. I cut a rectangle out of it and put it on & screwed the accordion shut. No more air leaks!

I was skeptical but the sheet was cheap and since its cut into 1 piece there is no place for air to come out.

Plus there are lots of colors to choose from

Dave