My Nukeproof has a Fox Float RP23 Boost Valve rear shock. I’d like it to be more progressive, but nobody sells volume spacers for my old shock anymore.

I’ve found this on Chain Reaction

Boost Valve

The chamber is a lot smaller than the one on my bike, would this make my shock more progressive ? Could I simply unscrew the sleeve from my shock and replace it with the one above ?

You could put oil in the air can or perhaps rubber bands.

Rubber bands eh ? That sounds like a cost effective and tuneable solution. I shall look into this, thank you.

I used the rubber strips that came with some lights