Couple of thoughts, observations, and ramblings on your 'topic', in no particular order.
The Hobart paintball 20 oz CO2 cylinder kit had 20 oz CO2 paintball cylinder, a fixed orifice 'regulator' on a short hose (with fittings on the ends to connect to the CO2 cylinder and the back of the welder), and a couple of 'brackets'.
The reported (fixed, non adjustable) flow rate on the 'regulator' was supposedly about 20-25 cfm flow rate and supposedly gave about 40 minutes of usage time on a 'full' 20 oz CO2 cylinder. But no gauges, so the only way you knew you were getting low on CO2 was when you ran out and your weld instantly looked like bird doo.
CO2 can be used to do short-circuit transfer mode GMAW on steel, just like C25 shielding gas can be, but the operating parameters and 'performance' are a little bit different from when using C25 gas. The pure CO2 arc runs a little bit 'hotter' than C25 (required ionization voltage for pure CO2 for a stable arc is about 10% or 2V higher voltage than when using C25, dependent a bit on the electrode wire diameter in use), and hence can give a little bit more penetration than when using C25. Which is pretty much the opposite of what you usually want when welding thin sheet metal with GMAW. It also usually gives a bit more spatter than C25.
And how much would it cost you to get a cylinder refill on that 20 oz paintball cylinder? Some google-fu says that a paintball 20 oz cylinder refill usually runs about $5 or so.
But the same google-fu says that to refill a 20
POUND CO2 cylinder is about $20-$25 or so. So that's 16x the CO2 for 4x the refill cost of the small 20 oz cylinder. That's 4x less expensive to refill that large 20 lb tank versus the refill cost of the 20 oz cylinder. Just saying.
Or you can just go and get a 20 or 40 CF C25 cylinder and use that. Use the welder's existing C25 regulator and hose and weld away. At a gas flow rate of 20 cfh, the 20 CF cylinder would give you one hour of gas flow time, the 40 CF cylinder about two hours of gas flow time. And the gauges on the regulator would let you see and know how much gas you have left and make (slight) adjustments to the gas flow rate if needed/desired. (no, you can't just turn up the gas flow rate on a 'small' MIG machine to 50 cfh trying to get 'enough' shielding gas to weld outside in a breeze, that high of a flow rate on the 'small' machine's torch nozzle will probably just give you extreme gas flow turbulence and make the effective shielding gas flow on the weld even worse.)
As to how to 'make' your own CO2 portable 20 oz paintball cylinder shielding gas kit for use with a welder, you'd need:
- CO2 cylinder
- source of 'welding grade' CO2 to fill and refill the CO2 cylinder
- fittings and hose(s) to connect to the CO2 cylinder and the regulator and the welding machine
- gas flow regulator
- bracket(s) or caddy to hold the gas cylinder
Doable? Sure. But I'd personally just use my existing gauges and regulator and a 20 or 40 CF C25 cylinder for 'portable' situations.
.... Like trying to get into racing and buying a Top Fuel dragster when you only have the money and knowledge to have camaro or mustang