Disposable helium tank (the ones I have are Balloon Time)
1/4" OD compression nut with insert
1/4" OD tubing
3/16" drill bit
5/16" drill bit
Disclaimer: Author is not responsible for loss of life, limb or property. Author is not responsible for anything. In fact, author is completely irresponsible. I mean, you should see some of the stuff he gets up to. It's amazing he hasn't burned the house down yet. Don't listen to him. Seriously.
Step 1: Cutting
Step 2: Drilling
Countersink with a 5/16" bit to a depth of about 1/4".
Wrap with Teflon tape, overlapping the edge.
Step 3: Attaching the Tubing
I've tested this up to 70 psi. Not sure how far it'll go.
I've done this with two tanks so far. On one, the valve doesn't close all the way anymore, so I put an inline valve on the tubing instead. The other one seems to work fine.
I'm currently using one for compressed CO2. Haven't decided what to do with the other yet.