My first holder for wet plate was an old dryplate holder, similar to what you've described. The truth is, it was terrible for wet plate because of all the porous surfaces. I varnished everything, but that didn't prevent a lot of issues with edge contamination. Once I switched to a converted modern holder, my problems disappeared. If you can disassemble the holder, remove the cardboard septum, and create a new septum out of plexi or aluminum to hold the plate, it would be 1000% better. I'm in the process of moving up from 8x10 to 11x14 plates and found a used modern Fidelity holder for <$200 that I'll be converting. It's a bit banged up, but should be good for a conversion.