I want to yell LEGOS ... but they may not be that feasible. hmmm now going on a google search for battletech lego dropships!
looked, lots of very cool sets.
But also some old ones if you can find them: They had a bunch of lego star wars planets with mini-figs ... I've got Yavin 4, might be a good size.
'course the Styrofoam might come cheaper, and easier to get to if there is a craft store near you.