The idea's yours, so not posting my own thread, Dean. Here was today's attempt at a drop checker. It's for my nano so I used the smallest 1/4" tubing that TAP carried in store. Figured why have the whole thing in the tank detracting from the scape, so came up w/ an ADA thermometer style instead.
That's a 2.5 gal for size reference. In hind sight I would have plugged the end, then made the bend, all before cutting it to actual length and making the final plugged end. I used pvc glue which made a nice clean and clear bond w/ the rod, but it didn't like the heating done nearby afterwards.
The small surface area of the opening isn't going to show CO2 levels quickly, but I have DIY CO2 on this tank so it's constantly on. Haven't decided if I'll use it permanently, but it'd work as a decent reminder when to change out the yeast mixture. I'm sure someone more adventurous here can come up w/ a way to make a more traditional version out of acrylic.