1. I notice the commercially available reducers (from Centek) are all fiberglass. Any reason why fiberglass is the preferred material to use in this application and not stainless steel?

2. Is it feasible to DIY fabricate such a fiberglass reducer, or is it likely to be more tricky than it looks? I assume one needs a resin that can withstand reasonably high temperatures for long periods, but other than that it seems pretty straightforward?

Thanks for any help!

P.S. I wanted to post a picture, but am also getting the 'Not enough space on the disk' error.

It's a lot easier to tighten the hose clamps on SS than on fiberglass which would deform/crack a lot easier

Cut a short piece of the old 3" (non-corrugated) hose, put it on the muffler, then ran the 3.5" right to and over it. Double clamped, no leak. The old 3" acted like a spacer. And if it did leak, it was already down in the bilge. 

Back to your question about fiberglass vs. stainless steel

I'd figure it's got to do with corrosion resistance in salt water as much as anything else and since you use your boat in fresh water, brackish water and salt water at different times, that's why I was wondering what your picture looked like.

SS may be good in fresh water like most people on here are used to, but there's a reason monel is preferred in salt water cooling systems instead of SS of any kind.

How long have your SS reducers lasted so far and what kind of condition are they in?

If you're having a hard time getting the hose off the SS, it'll come off the fiberglass with about the same amount of wrestling and probably destroy the fiberglass while doing it.  It's a lot easier prying on the SS than prying on the fiberglass till it falls apart.