There were two reasons that I decided to make a test stand; one was to break in the engine, and the other was to give me something to do while I saved money for a helicopter. The main goal was for the test stand to be cheap and simple, which it was. In total, I spent about $4 on it, and the design isn't too hard.
I secured the following parts:
1. A used engine mount from the hobby shop junk box, 1.25 inches between beams; $1
2. A used fuel tank from the same junk box, approx 10 oz; $1
3. A stock raptor muffler; donated from a local expert (a gentleman and a scholar, I might add)
4. A scrap piece of wood, in my case, house siding, from the same local expert.
5. A few bolts and screws from around the house; prepaid... just take down a few family pictures if all else fails
6. A piece of 7/16 dowel for legs; $1.50
7. A throttle linkage; also free, I just found it around the house.

As for the construction, I just sawed off a 2 inch section at the end of the siding, and attached it to the end of the stock with 3 inch deck screws.
This made something that had the profile of a capital L on it's side. I turned the engine mount so that the piston moves paralell to the horizon, so that the muffler will be in a convenient place.
I attached it with a few #6 bolts, and drilled two 7/16ths holes in the "firewall" for fuel line routing. A 1/4 inch hole made a nice throttle linkage route.
I mounted the engine to the mount in the usual way, except that I only used two in the interest of convenience.
The fuel tank mount was just four wood screws and two rubber bands, with a rag underneath to inhibit chaffing.
I drilled a few 7/16 holes in the bottom for the legs to stick in, and put an 8x6 prop on the engine.
If I was motivated, I would have applied a coat of clear polyurethane to fuel-proof it, but as I said, I don't intend to keep it long enough to justify and evening of curing.
That is about it, check out the pictures and I'm sure you can make one of your own and improve on the design.
If you want to buy it, send me an email. Make me an offer I can't refuse.