This is what I was thinking, using the base of the container to hold the spring in place. I was even looking at some 22mm plasic piping but I would have had to somehow open the inside diameter up to 21mm and leave the last mm original to act as a lip and I'm thinking this would be nearly impossible.
Delrin guides aren't going to work as the guides are built into the piston and trigger housing.
Perhaps the best solution would be to source a correctly sized spring?
A quick question if I may, does a spring's diameter increase when compressed?