hey john, sorry i didn't get back on this thread until now.
Foam cores: All very well, makes a very light and strong panel however its not so great in compression down the line of the laminate unless your skins are really thick, which then defeats the object of having your foam core. Also, my experience is that its not as easy to work with as plywood. For this application, you may as well use plywood as it'll be stronger, easier to work with and probably cheaper with just a bit of extra weight.
As for extending the walls of the console...wasn't part of the reason for the box was to fit the the fuel tank underneath? Anyways, I think I may be difficult to do it, let alone do it with a strong join without a lot of reinforcing inside (I could be wrong, I've never actually done it). Remember that there will be some pretty high shock loads from wave impact going through the console walls under your ass both sideways and in-line with the walls (which will want to split and buckle the joint). It will however (if done right) look nicer than a box under your console.
If I was going to do it, I would stick with the box method but I would try and match the curves of the console or something. Would be a pain to do (plywood doesn't bend round tight corners so well) but I'm keen on detail
After covering the outside is glass I would finish with filler (resin mixed with micro balls) and then coat with flo-coat using a medium density foam roller to give the "orange peel" effect.
I can't quite work out whether this post would have helped or not