I had an empty bottle that test strips came in and converted it to hold a test tube. Since the bottle was plastic, cutting it to the length I wanted and adding a tie wrap was simple. I tucked a couple pieces of small sponge on either side of the test tube to hold it in but still be easy to remove for refill/change out of the fluid. Keep in mind that tank water has to be exposed to the air inside the larger bottle as well as the fluid in the test tube. The changes in color are slow but I've never seen one that was fast either.
Here is a very fast drop checker design: Magnet-attached Drop Checker