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.
I found they worked merely okay. Maybe I just didn't have them positioned correctly but it was a little tippy. This was on a lily58, so the size may have made good balancing a bit tougher.
Either way, I switched them out for the magfrotto tenting, which works super duper well
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.
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.