Here's a BC I just made out of a glass tobacco bubbler from the gas station/paraphernalia emporium down the road, they sell various ones from around 4 to 10 bucks. A tad pricey compared to other DIY b/c's but looks better IMO.
This is what it looked like, glass vile, rubber stopper with two holes,
Removed the existing tubing and hardware from the stopper.
Cut CO2/In tubbing at an angle, dabbed with dish soap, and threaded it through a hole in the stopper about 6". Moved the tubing back and forth in the stopper under tap water until the seal tightened as the soap rinsed away, pulled back and trimmed tubing to target length, around 2.5" for this particular vile.
Obviously, the same process can be used for the CO2/Out tubing while trimming close to the bottom of the stopper, but I chose to plug in a standard tubing connector so the CO2/Out tubing can be easily pulled for quick disconnect. It's actually a connector from a Red Sea limewood diffuser that was laying around, the flanges on the end make a nice seal in the stopper.
Filled with water to submerse incoming CO2 line and corked with stopper. Make sure the rim of the vile and the stopper are completely dry or the stopper will not set a seal properly
Connected Outgoing CO2 line, finished. Cranked the gas, soap tested, all systems go.
Over pressurized the system for approximately one minute, uncountable bps and super microbubble storm, no sign of the stopper loosening. I'll let you know if it pops.
Total build time, a few minutes.
Now I'm off to find/make a clip or clamp I can mount it with. It works well dangling but that annoys me when I'm trying to count. It sets down by itself but will get knocked over eventually, doesn't weight much at all.