A-Zone.
The weather strip I refer to is designed to go around a door to stop the drafts. It is not rubberized, but is a grey coloured material like dacron? The building supply dealers have it, and I think they call it stop draft.
It is round, and comes either in packages, long enough to go around the door, or sometimes in bulk. The building supply I get it from has it in ½ inch diameter, which is just right for a 45. You cut off chunks about ½ inch long and it will easily squeeze into the case, on top of the powder.
Johnn Peterson told me about it and sent me a sample. But he couldn't get the ½ inch size where he lives, so he used the 5/8 size, but had to cut a vertical vee out of each piece, to get it to squeeze into the case.