How best to insulate betwixt hot and cold in a composite boat

The situation:

I must run a 3” diameter hot air duct “near” an icebox in a new custom boat build.

Boat is foam/glass/epoxy. Designing and building at present.

Let’s say the distance betwixt the pipe carrying the hot air is about 8” away from the inside border of the icebox.

What is the best way to PREVENT THE BTU’s from mixing up.

Is it 2-part pour-in Polyurethane foam? Is it baffles of very thin plywood set in series perpindicular to the heat flow path? A combination of these, or something else?

Bonus points: The hot air pipe, at 8” away from the icebox, will also be 8” away from the hull of the boat, which on the other side of that thin wall will have 49-51 degree F Puget Sound water rushing past at 9 knots. Heat suck on that side? Yes, foam and/or baffles there, of course, but center the pipe betwixt the refer and saltwater?

Ideas welcome...

 

Pipe insulation is not exactly a new concept.
A google search will turn up many results.

 

You seem to have sufficient room to use conventional solutions, to the heat exchange concerns you have. Is there a reason you're thinking you need a different approuch?