5gal keg?
Cheap off Craigslist ($25-50). Usable as-is with a standard keg tap. Modifiable to near perfection for a couple of hundred $ if you’ve got a fabricator you work with regularly
Edit: 4” triclamp is a really tight fit. I don’t recommend going smaller, because:cleaning, but it’s not a connection you want to open and close on a daily basis.
Edit: if using a standard keg tap, you probably need to disable the check valve on the CO2 side so you can pull vacuum. Certainly some clown seems to have removed them on all my keg taps 