I would just build it out of 2x4's and a 2x10 across the top. I have one that came with my Honda 7.5 and it works very well. Even works with my 25hp Johnson.

Basically it is a flat platform with 4 casters on it with 2 upright 2x4's that have a brace to hold them verticle. Then they have a 2x10 across the top to attach the motor to. With the 4 casters on the bottom it easily moves around my garage and it is strong enough to hold anything I would ever need to put on it.