I need to make a top for mine, but what I have is a 5 gallon propane tank with the top cut off and a open/close valve with an air line fitting installed near the bottom.
To cut the top off, you need to get an empty, prefferably new and never filled one. If it's used, empty it and remove the filler valve fittings so it's open to the air. You could just blow it out with an air hose to make sure it's empty or fill it with water. Then cut the top off. You'll need a big bandsaw to do it without screwing it up though. I have a piece of 1 inch thick lexan for the lid. I need to have a circular groove cit into it, so I can run a bead of silicone around it and have a tight seal. I also need to pick up the venturi pump from HF. This is for a vacuum chamber. If you want a pressure pot, I'd recommend buying one. It's probably safer that way.
EDIT: I'm not 100% sure about propane fittings, but all flamable gas cylinders have LEFT handed threads. It's so you don't screw the wrong regulator on a flammable tank. I figured you might want that info, so you don't wrench it until it snaps off
Raygun