A recent trip to a local shop called Creation Station turned up the usual bits and bobs for adding details and texture to terrain projects as well s a few unplanned finds. Prime among these finds were a couple of types of small plastic boxes, originally used for some sort of medical testing equipment their outer appearance looks like a good start for a science fiction building.
Inside one of these types of boxes, I found a few useful extras. There is a holding rack as well as two removable dividers. Though the rack is not really suitable for 15mm (holes too big) it might make good decking for 28mm or perhaps even some sort of dungeon grating for 28mm fantasy. If anything, they could be used as the basis of some sort of platform with a new surface added on top.
The boxes are nicely hollow inside and the top section that would be the roof, is hinged to allow it to open. I am not sure this function would be able to remain after conversion into terrain, but if it were you could use the interiors for storage of miniatures or even detail the interiors of your buildings. There are some nice scifi arches and texturing that would lend well to an interior space.
The second kind of plastic box was less interesting that the other. Though the top comes off there really isn't a usable inside. The overall shape is very basic and usable for a building with the addition of a few basic features like doors and such. I only picked up one of this kind, so I didn't have a change to test if it would stack well or not.
If you have any local surplus junk shops or anything you may be able to find this sort of thing. At 2/$1 how could I pass them up? That's about 1/4 the cost of plastic electric junction boxes.