i made one with bamboo canes ( froma garden centre)for the uprights and cross bar and cable ties ( zip ties) at 2 cm intervals up the uprights to balance the cross bar on. The uprights were each held up by being placed a hole drilled in a block of wood, then cross bar was balanced onthe cable ties so if some one hit it it would fall off. I put blobs of foam onthe tops of the uprights to protect any one falling on them, but they would fall over if they were hit too. I’ll see if I can find any pics to post later.
For best results your uprights should be as far apart as possible. This will protect them from the numerous bails that will happen during the course of a competition, as well as protecting riders from whatever is holding up the bar. One simple way to build one is with wood and a lot of brads (headless nails) to support your jump bar.
Always jump from the side where pushing on the bar will make it fall. Jumping the wrong way can damage or break your whole high-jump setup.
At Unicon X in China we used a regular track-and-field high-jump rig. It seemed to work fine.