LOL,i've got about 6 hours into this cover now,i'm thinking your foam idea would be the way to go
Getting the cover to fit tightly is taking a bit of grinding / welding and fabbing,turning into more work then i orginally planned.
The bend towards the fender,is a compound type bend,so it complicates things somewhat,plus having to build a couple oddball tabs to seal up where the box bolts to the fender.
This is the first and last one i think i'll build,aarrgghh,lol
Now i'm thinking of using this as a mold,and duplicating it in fibreglass or something,lol