I use a rope with loops on both ends. Dowel rod through the stear tube and loops on end of dowel. Can flip the frame and hang the loops on dropouts on my frame. And an old yard swing frame with plastic on top incase it rains while it's drying.
found a new way ... using a old stem in the seat tube, hanging on a post. when its just tight enough you can rotate the frame with a finger inside the crank or steering tube ..it can also rotate around the post , so you can have it in any position desired
the usual way ... i run a piece of mechanics wire down the seat tube and wrap the wire around a socket extension/screwdriver/bolt/paint paddle going through the bottom bracket.
Easiest way I found to paint one, is stick a piece of pipe in the ground next to something
that won't move. Clamp a 1'' wood clamp on the side and then stick either the seatpost
or the BB on the pipe. Has enough friction to stay in any angle.