1. PVC Pipe big enough to store what you want to in it.
2. Solid end cap at one end and threaded at the other, teflon tape or teflon pipe dope on the threads.
3. Drill and tap 2 1/8" pipe thread holes on it, one for a shrader valve and one for a pressure guage.
4. Cement your ends on correctly and let them dry till they no longer have any odor/fumes after being closed up for a day or 2.
5. Do a practice fit with the screw on end and pressurize the tube and let it sit for a week and watch the pressure, if it doesn't drop it's air tight.
6. Liberally spray the gun down with the gun oil you like, sock it, seal it and take it to a car dealer that has Nitrogen in a tank (It's like .02% humid) and pressurize it. If it makes you feel better about it drop in a bag of dessicant with it.
Keeps ammo nice too FWIW.