Fiberglass is likely the better solution. Why use duct tape? Why not use some thin sheet metal?
I've done it on some small spots that aren't in any way structural. Clean off the loose stuff, smear it up with (I use Hamerit) whatever and either pop rivet it flat or clamp and spot weld some around the edges. Then smear over the whole thing, both sides, with POR or paint or whatever. The trick is to knock off as much loose stuff as possible with a wire wheel in a drill or something similar so you get a decent bond. Fiberglass needs a really clean base to bond, paint needs less of a clean base to bond, but needs a solid base.