.... thinking about some lighter-weight alum on the outside with a round or square tube frame-work ... but then what would you suggest to insulate THE BEST as far as from the heat and noise ? ....spray-on, glue-on sheeting, several layers of each?

believe its called "DYNA MAT" Doc....

i don't know the name of it but in the HVAC-A/Cond industry they use the silver lined dense foam thats about 1/4" thick sheeting. it's used to line ducting & plenums to prevent them sweating. the stuff i used had the peel back adhesive surface that when applied to a smooth clean surface has stuck so well it'll be a b!tch to try & remove. very durable silver lining prevents oil impregnation & it's resonably flexible & there's a matching tape if you want to cover your joins for the finished look.