Update time!
Paper doesn't work to make a drop checker. It doesn't absorb enough of the fluid to change color enough to see the color change. I used some art paper, acid free, supposedly highly absorbent, and pretty thick. It bombed!

So, I thought some more and decided "SPONGE!!". I rushed out an got a pack of the little white sponge wedges women use to apply or remove make up. I cut a thin slice of one, fitted it into a membrane drop checker between a piece of Tyvek and a piece of Cole Parmer membrane. Then I soaked it liberally with 5 dKH water, and added a couple of drops of pH reagent. Then I sealed it to the end of a small acrylic tube, about 1/2" in diameter. Placed it in the tank, and it worked! It changed to green in about 5-10 minutes, is still tracking the color of the ADA style drop checker, and it was pretty easy to make. With some more work this could be a good way to cheaply make a fast reacting DIY drop checker.

Just out of curiosity I soaked a piece of the sponge in some distilled water for an hour, then checked the pH and KH of the water. Zero KH and 7 pH.

Next result: Tyvek Priority mail envelop material works fine as a membrane. It is thinner than building wrap Tyvek, so it is easier to use, and the cost is right where we DIYers like it to be. Of course it isn't transparent, so either it has to be a single sided membrane device or has to have a transparent membrane on one side.

Another result: I kept a 1/4" thick cylinder of fluid membrane device, with a single Tyvek membrane in the tank for 3 days. It never stopped working. This was just a 1/4" long piece of 1/2" diameter acrylic tube, cemented to a flat piece of acrylic to make a cup, with the fluid in the cup and the membrane held on with an O-ring acting as a rubber band. Easy, small, accurate, reliable, but no faster than the ADA style drop checker.