Well lets get scientific with this! You want to chose a material that has a high thermal conductivity (transfers the most heat) and is corrosion resistant....
here are thermal conductivities for various materials... there are numerous other resources that are more specific, but for general material selection, this link will work..
Aluminum is very high, at 205
Stainless Steel is pretty low, at 16
Steel is still pretty low, at 43
Aluminum would be your best bet as far as heat transfer is concerned.
But take care in choosing hardware... If you mate aluminum and stainless steel (common marine grade hardware) galvanic corrosion will set in and hurt the longevity of your project. I've heard various opinions on mitigating the reaction, but it seems there is no way of completely escaping the inevitable corrosion caused by the flow of those dang electrons. Various methods of insulation should be considered and investigated. Some say "you will be fine", but i don't trust anyone anyways.
Some things I would definitely determine....
How hot do you want it to get, how hot "CAN" it get (get a thermometer up in that compartment man!)...don't forget that the "fins" on a heat sink/heat exchanger can also be used to heat something up... (im imagining some sort of V8 powered jetboil camp stove....
) Heat transfer really is a beautiful thing...do some reading on Convection and Conduction as they will be the modes you deal with. Its cool info to retain anyways
Can you weld? MIG or TIG? How much do you want to spend (I've always wanted an excuse to "need" Aluminum fasteners)
I glanced at this and am saving it for future reference, has some good info on designing your part if you chose aluminum, like i would want to
BUT... if you dont want to have a weiner roast or fry pancakes on this thing, which would be awesome but I forsee luke warm weiners (no one wants more of those on the boat) and pancakes that get "eaten" with a straw.....instead of building a bulky clunky corrosion ridden thing-a-majig to mount on your motor....why not just build a rack and mount it to the bottom (inside of the engine compartment of the sun pad/engine cover to hold your items to be warmed? Hopefully you could utilize hardware and holes that are already there! I love when I dont have to add more holes to things....
just another idea to throw into the jumble