This might work.

Take an old seat post weld or bolt it to another pipe (about 2 feet long).forming a “T”.
Just remove your seat post and install the old post with the bar on it.
Clamp the bar in a vice or drill a hole in the wall or a tree, (improvise) jam it into something.

…tie it down.
…nail it down.
…wedge it in a tree.

Who knows?

i think i might build a couple of these and throw them under the back seat.
…might come in handy some day.

Buy the best tools you can afford.
In the mean time do the best you can.