Would anyone know how to build a good device to balance a prop (home made) other than a nail?

Attach two single-edge razor blades vertically to something such that the sharp edges are parallel and level, and there is room for the prop hub between them. Use a short drill rod or piece of metal tubing to fit the hole in the prop. Let the rod roll on the razor blade edges. If one side of your prop goes down, take it off and reverse it to see if the same side will still go down or if your blades are slightly out of alignment.

There was a commercial gadget like this that used double edged razor blades bolted to a piece of aluminum channel.

Jim

One of the senior guys at my club mentioned that balancing a prop at the .40 size is pretty much pointless. If you're running bigger engines, i guess you shoudl though.

I was using a 3 bladed prop on my .45 and it was unbalanced. The main gear bolts needed tightening every 2 flights and it finally shook 3 engine mount bolts out. I landed with only 1 bolt holding the engine. Balancing the prop cured the problem. It isn't hard to do and I've seen about 1/3 of my new props out of balance.

The Finger Balancer by Great Planes is pretty good for a beginner. Put it between 2 glass drink glasses. That is very sensitive and works well. The Finger Balancer is something like $7.