I wound up attaching an airline clip to the suction cup that affixes the bell to the tank wall and then clipped the airline from the DIY CO2 bottles to that. As a result, no problems with the loaching tipping out the CO2! So thanks for the suggestions -- it's working great!
I'm considering, though, getting the Red Sea Turbo system
just to get the Venturi pump-driven diffuser/reactor thing from it and using that instead. The quiet in the room is most important after lights-out, and so I was thinking about running the pump on a timer opposite the lights -- it'll be quiet at night and diffuse less CO2 without the pump on, hopefully it won't be too noisy during the day. Most importantly, it'll be smaller than the bell. (As I said before, the bell isn't totally unattractive, especially once the plants in front of it grow in a bit more, but since my 29 gallon is only 12" front to back, that 3-4" across bell takes up some planting real estate that I'd rather leave open.)
I figure I could use the small yeast/sugar/water bottle that comes with the Red Sea system to power some DIY CO2 in my nano tank, just bubbled in with an air line and a limewood airstone or a Hagen ladder to keep things simple, then nothing would go to waste.
What do you think? Good idea? Bad idea? But whatever I decide, at least I now have a more stable DIY system in place for the time being!