I used four pieces of insulated electrical wire, maybe 14 gauge or so. Over that and the guide I slipped on a section of 1/2" adhesive lined heat shrink. Makes for a tight fit and has held up well.
Polish the ends of your spring and put a bit of lube on it. You might also consider a sleeve in the piston to isolate the spring from it.
Or even trim a couple coils if it's extra long with a lot of preload.
Rear spring guide for a Gamo Accu or CFX or any of the other marketing names is a simple matter. One can be turned up out of Delrin. It can be turned very well on a wood lathe and is not precision engineering. Just make sure that the fit of the spring over the guide in the non compressed state is firm to very firm. Delrin is cheap so even if you have to make two or three to get it right it will not break the bank. And you can do it yourself. ( Lathe might be a problem to some but maybe a friend or even a friend of a friend has one.)
Sorry to hijack. Does it twang like a big cat? I had one that twanged so loud I couldn't hear ametal silhouette being hit.
With my wire/heatshrink, plenty of retired time to redo it mod, zero clearance. The spring rubs the shrink tightly as the guide is lightly forced on.I also sleeved the piston with plastic, lined the tophat guide with non-adhesive, 1/2" heat shrink and stuffed an old pair of socks in the butt stock. I spared no expense. Literally. Zero twang. Nada. Sounds well mannered and downright polite when I shoot it.
Could always could covert it to a gas ram to get rid of the twang:https://www.customairseals.com/product/gamo-ats-sat-trigger-gas-ram-kit/