I built mine using a Great Planes 60 size engine mount attached to a piece of hardwood a little bigger than a typical firewall and screwed/epoxied that piece of wood onto a 2x6. The engine mount has holes drilled in it to accept 61 four stroke, 70 four stroke, and 91 four stroke.
The fuel tank is attached behind the "firewall" with a small bungie cord. I have run quite a few 91 and 70 four strokes on it and it works great.