You? Wrote
["Which raises to question, why a diesel heater cannot just heat the water directly. Everything I have seen shows diesel heaters being used in conjunction with coolant of some sort specifically, and not just water directly].
[I'm not sure what a plate exchanger is, so I'll have to...
One approach is to make your light wooden frame with studs/stringers, exactly the same wall thickness dimension as your foam thickness that you decide to go with.
Using epoxy glue and screws, cover the light frame with ply, then fit & glue in foam panels from the inside, then line inside of the...