I spent about 2.5 hours playing with acrylic tubes and a heat gun this morning, and my 8th attempt at a drop checker is the first that's worth showing. It's probably too darn tiny to actually be useful, but here goes:
It's misshapen and has bubbles, but it seems to be airtight. This one was made from 1/8" ID tubing with 1/16" walls. I sealed off the "sphere" by cutting the tubing off with a thin dremel cutting disc, sanding a flat spot, gluing a piece of 1/8" acrylic over the hole, sanding to shape with the dremel, wet sanding with successively finer sandpaper and finally flame polishing.
I have been unsuccessful in replicating this with larger diameter tubing, but I'll have to wait to try again. By the end my cheeks were very sore from blowing into the tubes. I have found that less heat and more pressure seems to work better. More heat tends to produce bubbles in the acrylic that make thin spots, which blow out easily and ruin the piece.
Just a note: I actually prefer brohawk's and aquadean's designs, but I am trying to make one for a friend who has a Fluval Edge and can't use an over-the-side style.