Just a thought, but if the membrane idea pans out, these could be made even easier by gluing a piece of membrane to the top of any small clear container filled with indicator solution.
Like this:
It wouldn't matter which way was up since water can't go through the membrane and you could use any small clear container you have lying around with virtually no modifications. Heck, you could just glue membrane to either side of a piece of air line tubing. Then if it starts going bad, toss it out and throw a new one in.