Just make it a doodad with no pathing and stick it over some terrain to cover the terrain up. Then you'll be able to stick units on it and all, no problem.
ull have to make a ship out of doodads, not very sea worthy but fine for cinis. it wont move though thats the only problem and u cant make one that does like a unit. u could move things around it though... messy but it would work.