I did something similar once. I used a piece of thin-walled aluminum tubing that was attached to the fuse. It was 1/2" I.D. tubing, but the muffler exhaust was only about 3/8" O.D. so it had about 1/8" of clearance. Being attached to the fuse held it in place.

There was no power loss at all.

Now, on the other hand, I have used those silicon DuBro muffler extensions and suffered a noticeable power loss.

So I don't know if (On the aluminum system) it was the larger diameter, or the openings at both ends or both, but it worked like a charm.