I simply take a piece of 3/4ths inch thick wood and with a coping saw, cut out a a portion, so the base of the crank case of the engine will fit between. Like an engine mount, the chunck of wood ends up shaped like a two pronged fork. The wood piece is long enough, so I can secure a tank to it and use C-Clamps to secure to a saw horse or bench. I drill holes through the mounting lugs of the engine and bolt the engine to the wood mount. I use rubber bands to secure a fuel tank onto the wood mount and C-clamp all to saw horse or bench. I also use a metal push rod to control the throttle.

Dirt simple and cheap. Many years ago I built an adjustable one, used it a couple times and I have no idea what happened to it. Also, it wasn't quite adjustable enough when four strokes started coming on the scene.

Sometimes bench running really isn't needed. Mount to the plane, run a tank through on the graound and fly the plane is all that's needed. However, to familiarize your self with operating, they are a nice way to go.