Alistair,
I've missed this Thread until just now, beautiful work on the interior and the engine! As for the foam question I have an assortment of the stuff salvaged from the packing of various purchases over the years; tools, electronics and video stuff, glass ware etc. are great sources and offer a variety of densities. Most of it is very similar to the liner Iwata uses in their airbrush cases for example. Works like a charm and the price is right!
Cheers,
Lance