Aren't GSU's engine tin gaps too wide for the Vanagon foam gasketing alone? I'm thinking he needs to get closer with some aluminum sheet and THEN the gasketing.
Yes you will still need side "tins". My CMC came with 1/8" fiberglass pieces that could be shaped as needed. The H gasket is ideal for 1/2-3/4" gap. One manf used wood grain imprinted siding for the tins, others galvanized duct work metal. I'd much prefer machine tooled polished aluminum.