Essentially make a sawhorse. then a ply plate with a cutout for the engine to fit in. (different ply plate for each engine) Add a snaked cable type throttle linkage. (the bends will tend to hold the throttle where you put it.)

Essentially... anything that holds the engine firmly will work. I do all of my engine testing with the engines mounted to an aircraft. the test stands are just an extra mounting and dismounting process. Tie the plane's tail to a post... and you can let the engine run for break-in.