ErgoMechKeyboards

I used books to put board over, quite adjustable, though not the best.

If it's cheap enough to you, the z-tripod is only 25$ (you'll need two). You could find even cheaper version were you just put the keyboard on it and rely on gravity to keep it from moving.

To give you an idea, manfrotto pocket tripod + magsafe adapter+magsafe rings is about 120€ (about the same in $ I would guess). To me that starts to qualify as expensive.

A very cheap version if you have a 3d printer (or a friend who owns one) would be to design your monoblock stand like this https://www.printables.com/model/311468-corne-tilt. There is the option to print legs too (https://cults3d.com/en/3d-model/gadget/corne-keyboard-tenting-mod). Jlcpcb or other manufacturers can print them for you (but mind the shipping and taxes).

Currently using these and they work pretty well. https://a.aliexpress.com/_mqz9UNE

I use these phone holders; cheap, sturdy, easy to tighten and fold down: https://www.amazon.co.uk/dp/B0BP1ZQTJ1

Depends on the definition of cheap, but you can find stuff on thingiverse and get it printed/print yourself if you have the hardware. Otherwise a tripod for each side seems to be fairly popular, either with gorilla grips or simply legs you sit on the desk.

Adjustable Cell Phone Stand for... https://www.amazon.com/dp/B08KDWXCK4?ref=ppx_pop_mob_ap_share

I got 2 of these and they work pretty well. I put Velcro on mine to hold it in a little better cuz I'm a heavy typer.

If you have access to a 3d printer, try looking for a tenting case on Printables or Thingiverse. The CRKBD printed case I downloaded was pretty good. Plus I was able to modify the design over time to suit my needs. My current iteration is over a year old and I love it.

Bean cushion/rice bag, anything deformable. Works really well. Buy two smaller ones for distance of you want.