Well, as everyone has brought it up, let's talk cartridges. These must be made out of metal or a chemically resistant plastic like polyethylene with a high tensile strength. PVC, while sufficiently strong isn't fully resistent to acetone. I will be making mine out of carbon steel or just regular steel. Aluminum is a good option if it is really thick, but most of us don't have the equipment or knowledge of proper welding techniques.
Now if you want to make rectangular cartridges, be aware that the reason noone makes them is that they have worse pressure limits. If you want to test the mathematic possibilities:
http://www.gowelding.com/pv/square.pdf I'm not going to lie. I can barely comprehend the first three pages before getting bored. It is alot of math for square cartridges.
Finally, pressurizing the things.
1.) make the pressure based off of a chemical reaction. The elephant toothpaste is nice but you would need 60 to 80% hydrogen peroxide and it is near impossible unless you distill it.
2.) connect a co2 cartridge to it. There is a possibility of leaking in a tube.
3.)Compress it. It requires a piston or air compressor. Heck even a helium tank or CO2 will work but you need to cap it off.
4.) soldering, yeah right. Unless you and your iron can move faster than 100 to 300psi, I don't think so.
5.)Check valve- clunky but accomplishes the task.
6.) any other valve- risk losing or gaining too much pressure. Also there is a danger with being too close.
