A brief update to this thread.

Last night I took a vintage paper filer (kinda like this one) and wrapped it with black plastic sheeting. By taping the plastic around the corners I can hopefully keep all the light out, and then a loose flap allows for loading it, using magnets to secure that once it's loaded.

My one concern is getting airflow throughout. I've got a sample of black Tyvek coming, which if it's sufficiently light proof should make a nice "breathable window".

The metal filer is appealing because I can use magnets to attach my carbon tissues, though they curl so strongly I've had to order neodynium magnets! The metal can easily be hosed down too, to prevent any dichromate contamination.

If the black Tyvek is sufficiently light blocking, it might be a good material to use in place of black plastic, since it lets water vapor through.

On the other hand, if it doesn't work, I need to find a better way to get air in and vapor out. Trying to think of some kind of simple light-trap, but so far I'm a bit stumped...