I found a pretty easy way to make a DIY drop checker for under $5. Well, two ways, actually. Here you go:
This is the one that I am using. My husband used to make wine, and there's this little gadget used on the fermenting containers I guess to let air escape from them. It's called an airlock and costs under $2 at brewery/winemaking supply stores. http://www.winemakersdepot.com/Cylin...lock-P151.aspx
This is the style you want. There's a plastic piece loose inside it that you can see in the picture. Remove that. There are also two little holes in the lid, so I sealed those up with some silicone. Then I siliconed a suction cup to it (available at fine stores such as Dollar Tree or Walmart in packages of several, or you may have one laying around the house unused) There you go. The airlock I used happened to have a line on the plastic partway up that is a convenient fill level marker for the 4dkH (I made mine at home, someone else on here already has instructions for that). Add that and your bromothymol blue pH reagent, and stick it in the tank.
Option #2 is for those who for who don't want to purchase the airlock. You need a small clear plastic container with a tight fitting lid. A craft store sells paint mixing cups that would work, or you could use a very small plastic jar of the type spices come in. It just has to be waterproof. You will also need about two inches of 1/2 inch clear aquarium tubing, like the kind used for pythons/gravel vacuums. Or a similar size, just not the really tiny stuff. Cut a hole in the bottom of the container the same size as your tubing, and stick the tubing in so that it is halfway up the inside of the container, and still sticking out some on the bottom. Silicone it in place well so that it is watertight, and silicone the suction cup onto the side so that it will hang on your glass. Fill it up partway with your 4dkH and the b. blue, and you're done.
Here is the finished product, option #1.