I live is Sweden too and I bought a SodaStream Co2 cylinder to harden waterglass mixed sand. They can be found in most well sorted super markets. ClasOhlson has them for 80 SEK, only sold in shops, not online due to shipping hazzard or some such overprotective babysitting nonsense. I think most ICA SuperMarkets sell them too.
The thread on the cylinder is a rather course affair, a 4mm pitch if I remember correctly, and about 14mm diameter.
A simple tool to bleed the bottle can be turned from aluminium round, threaded in one end to fit the cylinder and the other end to fit a m6 plunger that pushes down on the valve.
Drill and thread a perpendicular hole for a hose fitting in the middle of the tool, attach a hose and Bobs your uncle.
A suitable o-ring will take care of any exessive leakage between the cylinder and the tool, but do NOT make the tool gas tight. Do NOT add a second valve at the hose fitting.
If the plunger malfunctions and bleeds the bottle at full throttle you want it to gas out to atmosphere without building up preassure inside the tool.