there are now 3 versions of "great stuff" expandi-foam on the market, red
can is teh traditional, about 60% expansion, a blue can for use in window sashes
and draft apps, and the black can for filling large gaps [lesson
black=bigger]
to do the inside of the heater box, use the red can, it is the strongest,
it can be formed when the foam forms a skin, by using a wet cloth- not that its
water-soluble, but to prevent sticking to it- any excess can be easily shaped
with a hacksaw or sawzall blade, or sanded with 40 or lower grit paper. it
also works wonders to refab molded dashes, custom headliners, sound deadening,
headrests on modern [60's up] cars etc....