Source for 2600 cartridge holders for inside box?

I have recently acquired some third-party Atari 2600 game boxes, but they are missing the cartridge trays/holders that should be in the box.

I don't really want my cartridges banging around loose in these boxes, so I would like to find a workable replacement, such as the ones that come with many boxed homebrew games.

Does anyone know a source for these?