It seems to me, there's no good reason that the liquid inside the checker must be all liquid.

The sponge notion or porous paper etc is a good idea, it increases the surface area for colorimetric exchange and speeds up the phase into the liquid from the gas. You tend to get even coloration with less time involved.

Remember to take out the KH solution and run test of those as well to make sure the cheapy membranes are holding the water back and maintaining the ref KH.

I'm leery about those cheapy membranes, although I know that they can be used, I just want to be more certain.

Regards,
Tom Barr