Hmmmm....I am going to be mounting a headphone jack on the front, and RCA inputs on the rear. My concern is that the force of inserting and removing headphones/interconnects could either cause the sheet metal to be dented, warp or (in a worse case scenario) cause the headphone jack to rip out of the sheet metal entirely. So I wanted to add a sheet of metal on the inside to thicken the sheet metal of the enclosure itself, distribute the force of insertion/removal and provide a more substantial hold for mounting screws. From that perspective, I think a sheet is better than an I-beam, although I am happy to be corrected.
You think the silicone would do it? I like the temperature resistance, and the flexibility of silicone would presumably help prevent the bond from cracking/weakening due to flex in the two metal surfaces when headphones are inserted/removed.