IPD used to offer a hood insulation kit, and I got one. Maybe they still do - worth a look.

It consisted of pre-cut panels of adhesive backed thick foam with a foil facing toward the engine. The panels fit in between the formed metal underhood bracing, up against the underside of the hood sheet metal, so not a big one-piece sheet like the original. Still OK 10+ years later.

I bought the kit because it seemed like a good idea and it was fairly cheap. I can't say it made any difference, at least to my ears, in engine noise, but it does insulate the hood skin somewhat from underhood heat.
--
Bob: son's 81-GL, dtr's '94-940, my 83-DL, 89-745(V8) and 98-S90. Also 77-MGB and some old motorcycles.