You need a piece of all-thread 1/4" dia. about 8" long. Four 1/4" nuts, and two long 3/8" nuts.
You need some stiff wire. Two pieces about 24" long each. Bend loops on the ends that will fit the 1/4" all-thread. Bend one wire in half.
Run the unbent wire down the seat tube and put the all-thread bolt through the loop. Put one long spacer nut on either side of the all-thread and put a 1/4" nut on either side and tighten.
Put the bent wire loops on the all-thread and put the last two nuts on and tighten.
Hang it upside down and paint the underside of the frame first, starting up high and working your way down.
Flip the frame and paint the upperside of the frame last. Again, start up high on the frame and work your way down. This method helps to keep the overspray from messing up your paint job. I've used this method many times and it works well for me. Gary
The metal hanger will support the weight as long as you have it twisted together real good at the loose ends. I have used this method many times. I always try to find one of the heavier gauge hangers just to be sure!
I've hung them with a bungie cord through the headtube, with the fork pushed part way in. The cord takes up enough space that it holds the fork from falling out, while still allowing me to turn the fork if needed.
i hang mine by running a bolt through the seatpost clamp and use wire hanging from the ceiling attached to the bolt. then i use a push broom and run the handle through the head tube with the broom bristles on the floor, its straight and wont turn on you while your painting.
I insert a goose neck in the seatpost tube, slide a piece of gas pipe through it and hang it between two ladders . You can raise the frame by the head tube to paint the bottom and hard to reach places.