WELCOME to the Forum!!
When a kid, making my own jumps I would use objects that I could lean a rail against or on top of that if the horse hit/bumped into it it would propel away from us and release, fall to the ground quickly so as not to tangle in the legs.
A log, 2 of them cut the same height, a rail placed balanced on top.
Upside down bucket, again placed a rail on top so if hit it freely fell away.
I would place a rail balanced against a tree trunk...
I used tires stacked with a rail again balanced across the top...
Everything I used was able to be added height by stacking...but never, ever
did I secure a rail or put it between objects that held it in place preventing the rail from falling away if he hit.
My rails were always wood. Often a old fence rail from post and rail fencing or rounded edge landscape timbers... Never use plastic pipe
of any kind as hitting them if brittle can be deadly shards impaling your horse. PVC fencing is also a no-no for the same reasons
...not made to take a impact they shatter and can kill or maim terrible injuries for horse or rider.
Building a few single pole standards is cheap as are jump cups...you only need to invest a small amount of funds to have enough standards to make 3 or 4 fences that would give you many options for type of fence and heights achieved.
There are many do-it yourself designs on the internet today with step-by-step instructions..
Enjoy your project.