This is partly to show what can be done, and also a reminder to be cautious when playing around with pressurising things!
I decided to make one out of an out-of-date 4 kg gas bottle. These are tested to 480 psi, so are made from reasonable steel. First job was to remove the valve - I've removed three now, and it doesn't get any easier. Once that was out I filled it with water and let it sit for a couple of days (just in case). I then cut it in half, about an inch above the weld (suggestion: empty the water first!!). I forged two circular rings out of 1/2" steel bar to fit around each half at the rim and welded then in place, this was followed by a piece of thinner 1" stuff welded to the lower ring to make a rim into which the top half fits. Three clamps were made from 1/2" stock, using 3/8" threaded bolts as the screws. The clamps were pivoted on brackets welded below the lower rim. I made a rubber gasket to seal between the two halves, fitted a valve and pressure gauge and that was it.
First problem was my welding sucked! It leaked like a sieve, so I had to reweld most of the rim (still leaked a little, so I made a wider gasket, which seemed to work). Regarding the welding, the problem seems to be the steel that the tank is made of, it caused a lot of sputtering on the first pass (gasless MIG). I tried increasing the voltage, for better weld penetration, but promptly burned a hole in it.
Anyhow, there were only a couple of small pinhole leaks when I fitted the wider gasket, so I let it build up pressure. Got to around 55 psi and with an almighty CRACK the top half flew off and through the polycarbonate roof overhead! Very spectacular! (Luckily I was outside, under an awning, and not in the shed). Inspecting the damage showed that the welds on two of the clamps had given way. My fault, as I after welding them I ground them down to make them neater - DUMB!! The parts need to have a deeper V at the joins, so there is better weld penetration.
At the moment I've put it on hold while I consider my next step. I may forge the clamps out of one piece of thicker metal and then just weld the screw assembly on to each one, that way there are a minimum number of welds involved. A bit more work, but I have some 1" round stock that could be curved into the right shape. On the other hand, I may just reweld the existing clamps (properly), I only need to get to around 60 psi, so I'm almost there.
In short, a lot of work (but quite enjoyable) but also potentially very dangerous. I consider myself lucky, the results could have been much worse. So BE CAREFUL if playing with pressure equipment.