Don't see any reason why you cannot make one and pop-rivet or weld it to the exhaust. Should work fine. Go to a sheet metal shop with the old rotted piece and they can probably make you a replacement using the old piece as a cheap and nasty pattern. If they use galvanized metal it will be harder to weld.

Also, check out a custom car or street rod builder. There is some pretty neat foil covered heat insulation material hot rodders use that would work nicely if laid under your floor mats.

230/8