Re: Gas tank shroud
I think that type of foam would be the best method since you're covering an existing tank and you want something to follow the contour of the old tank exactly. Then it's optional to leave the foam in there after the glass cures since the foam will also help keep the tank cover lined up and centered on the tank without the need for fasteners of any kind.
One really nice thing about doing it this way is the ease of repairs if something goes wrong like a tip over it just means cut out the damage, fill the dent with more foam, contour match it, and apply a few more layers of glass and re paint. Keeping the foam in place will also protect the real tank from damage in the event of a crash where the last thing you need is 1/2 gallon of gas all over your bike and yourself.