There's a thread around here where someone suggested corny kegs as a pressure container. I picked up a couple of reconditioned ones, a connector set pair of valves, 1 foot of neoprene hose and 4 clamps. Got this lot delivered from http://www.cornykeg.com/
as the other poster recommended.
To the above I added one brass schraeder valve fitting -- like a car tire stem but threaded for a dust cap on one end like a stem but also brass threaded on the other end at a diameter suitable for sealing against the inner diameter of the neoprene tube. Got that at a local auto parts store for a buck and change. While I was there I got a $12 pressure gauge also with a screw thread that was sung for the neoprene tube.
Assembly involved making the inlet and outlet tubes using the connector set, gauge, schraeder valve and the neoprene:
inlet: valve - 6" tubing - keg connector (each '-' end of the tube is clamped with the hose clamp thingies)
outlet: gauge - tube - connector
(Mind you get the right color red/white connector for in and out if you try this -- took me two goes to get that right!
Hardest assembly part was getting the schraeder valve thread and the gauge thread seated in the neoprene...tried cold first but after cursing for 5 minutes I blasted the tube ends with a hair dryer for a minute and it went fast from there.
Total assembly time...maybe 20 minutes from start to first applying pressure.
The schraeder valve means I can use a tire pump or my compressor that has a tire inflation fitting to deliver the pressure nice and easily.
Kegs hold 40 or 50 balls at a time (haven't counted exactly but plenty for a practice basket for me anyway). Only real pain is that the opening in the top of the keg is a bit tight so getting the balls out is a bit more fiddly than just upending the whole thing. These things hold pressure very well for a week or two (and it needs to be that long for some balls; Dunlops lose pressure slowly but they take it back slowly too I discovered!).
I wouldn't claim that's an original since I copied the idea from another thread here but I can claim it's pretty darned easy to do...I'm not at all mechanical or handy.
Works pretty well. I have two kegs and plenty of old balls with decent felt still so I can always keep a couple of sets cooking and have some 'retreads' in the basket at the same time.