What about something like these
http://www.toolstation.com/shop/Grou.../sd3156/p49173 ... a concrete block and some wood bolted on for the engine to clamp to would give quite a solid base...
I gave my set away a while ago, but the plan for them was to put our 50 tohatsu on it... did put it on there for a bit, and seemed fine... just didnt get around to making somewhere for it to clamp to!