you might look for photo grade gelatin
at the photographer's formulary
it will make your life easier than using what
i tend to use ( food grade knox blox )
which really doesn't work too well ....
you might consider coating paper with it to make your test strips ..
process it in the same chemistry, the same way ... i think hardener might be useful too ...
( so you don't waste all your materials )
considering it is sleeves of cardboard ( before it is folded up into a jacket ? )
you might also consider an alternate way to process the print
a big piece of cardboard will soak up your chemistry like a sponge,
and using a sponge to process, might damage the emulsion ... your project is going to be fun ... and tricky
have fun !