Mine are just a bunch of 2x4's and 2x6 blocks screwed onto my home-made sawhorse. It's amazing what contraptions you can come up with culled lumber, a circular saw and a drill driver.

Whatever you do though, it's important that you elevate the engine to the proper working height. It's also nice to build a stand that you can reposition depending on the prevailing wind direction. Nothing worse than having smoke blown all over yourself when breaking in a motor.