I wrote about my experience with the FWB Sport here:
https://www.airgunnation.com/topic/more-observations-on-the-fwb-sport/#post-1033132 In my review I discussed the evolution of the spring guide that currently resides in my rifle.
I'm pretty sure I used mild steel, though it could have been stainless. I think mild steel is perfectly fine and I have no doubt that delrin will work very well also. The hardest part, I thought, was drilling that deep hole without the drill wandering. If I recall, I drilled part way then turned the piece around and continued drilling from the opposite end. It worked perfectly.
Here are the dimensions I used should anyone be interested. Beyond the increased diameter of the guide surface all measurements are the same as you would find on the original spring guide. Also, I did away with the three pieces as found in the original assembly. Obviously some of the measurements are more precise than they need to be, but I was trying to replicate exactly those dimensions found on the existing guide so I would have a baseline during the manufacture of my own. You may notice that I eschewed the .620 diameter measurement on my drawing in favor of .625. That gave me a bit of an interference fit between the spring and the guide. As I'm sure anyone who has ever done this sort of thing can imagine, I made it so that I had to kind of "screw" the spring onto the guide. Why FWB engineers allowed the inside of these rifles to be such rattletraps still baffles me when it really is such an easy fix.
Of course all airguns are slightly different, but I'd be willing to bet that these dimensions will work with all FWB Sport rifles.
Once I added this guide to the internals of the rifle, it completely eliminated spring noise while cocking. It's silent now, Stealthy, in fact. Also, as others have mentioned, it did away with the rather epic buzz.