Slant boards have become popular, and they are undeniably effective.
If you want a cheap alternative that can help you, go to your local hardware store and get a 2 x 4 piece of wood.
This will give you about 5° of hill elevation, but depending upon your body proportions, this can make a significant difference. You can see here that, for my wife, he gives her a picture-perfect squat technique and better biomechanics.
Using a 2 x 4 is an old-school tactic that has been around since at least the 1950s, but seems to have been forgotten about today.
It works, and this piece of wood probably won’t cost more than two or three dollars.