Washer help needed; look what I left the bar with last night

[VANDEEN]

For the princely sum of fish and chips, a couple of pints, and a lift home I.e. <> £13

A waist height CO2 bottle (out of test but still half filled) & this

Using the pipe from an old foot pump & a bolt in car tyre valve I cobbled this together

The regulator is missing a primary gauge, but from what I've read on here people don't rely to heavily on them for judging gas capacity anyway?
The secondary is within 1-2 psi of what the foot pump gauge reads and I now have a nicely carbed pop bottle full of water.

After my test I removed the reg. And found what I assume too be a load of plumbers putty to make it seal to the bottle. I've cleaned it all off now & guessed I needed a washer of some description.

So, Do I go fibre? Nylon? Silicone?
&
does it fit over the larger of the two diameters and inside the nut? (Like the trial one I made from silicone in picture 2)
Or
Go inside the bottle valve assembly and seal against the front face of the smaller diameter piece where the gauze is)?

Are parts for these reg's still readily available as I fancy stripping and checking it over?

[Haydnexport]

get one of these and some of this