Hi David,

I'll answer your question about modelling, as there are also ways to handle this in printing also (wall shells etc).

Yes there definitely are methods of removing model detail and filling volumes and the process is typically called de-featuring, however to achieve what you need you don't have to go through de-featuring the model unless you're already familiar with your CAD software and how to do it.
In your case I'd suggest a relatively simple way to do this, just create a volume inside your model and bound it to the model edges, if that still is too complicated send me the stp file and I'll do it for you - in fact we're about to run a promotion to offer a free basic CAD model to people that like and review our services (all you would need to do is like our facebook page, send me the stp file and I'll fix it up for you so that the insides are solid ). If you're happy with the stl file afterwards it would be nice to get a review also, but not mandatory.

Note the promotion will be limited to the first 25 likes, but it hasn't gone live yet so you're guaranteed a place