I built one: Went to home Depot, to the scrap wood bin. Took 1 pine 1.5*3" wood beam and some scrap plywood sheet. A couple of wood screws from my large one dollar store screw assortment, wood glue and a music wire (for throttle control). Took me half hour to build it. The tank (of any size) is attached by leftover Velcro. The engine mount (any size) is screwed to one of the parts of the pine beam. Two long horizontal parts of the pine beam are used as a base. 4 3/8" through-holes in them are used for pinning the thing down into the ground with tent hooks (optional).

If interested, I can post a picture. (Just how do I attach an image here - haven't figured out yet...) Made mine for a 40 size. Can do the same for much larger size.

So what you need:
1. scrap wood (see above)
2. dollar store wood screws
3. wood glue (optional)
4. wire
5. Engine mount (that you would anyway use for the specific engine)
6. tent hooks (optional)

Equipment:
Jig saw
hand drill with a Philips driver bit and 3/8" drill bit

Cost: 2 dollars
Time: 1hr (going to the stores)+ 1/2 hour(actual building)