Where can I find a plan for the ManWin Trainer (made from coroplast) for use with a Babe Bee engine?

Here you go.

The eBay 3d printed motor mounts break very easily.  I wouldn't recommend using them at all.

Yeah, this is a good place for recycled license plate mounts.

Worst design ever

  I never built one, but for a beginner that nonexistent tail moment could be problematic. Keep the wing. Build a flat fuselage from a hardware store yardstick, and add some sort of engine mount. Set the mount up for a Wizard style rubber band shock mount with about 3 degrees offset. Add 1/2 ounce tip weight. Bell crank can be mounted to the fuselage. Add a horizontal stabilizer with a nice shape and add the elevator from the plan to the trailing edge. Vertical stab not really necessary but something pleasing to the eye can be glues into a slot cut in the yardstick, along with some sort of canopy that can double as a handle to make an underhanded launch. The airplane as presented on the plan would be quick to build for someone want to do some sport 1/2A combat, but for a trainer, not the optimum, I don't think. Making all the required nose and tail moments match the Goldberg Wizard would be suggested also.

  Type at you later,
   Dan McEntee