If this is for a semi-permanent setup:
- get two rubber erasers (or corks from a wine bottle, as long it those made from real cork wood), these provide enough drag resistance so it does not slide too much.
- cut them to the desired height and angle. This requires some experimentation and tinkering. If the erasers are big enough this might actually be enough. Place them under the back sie of the keyboard.
- use a non too permanent double-sided adhesive strip to make this reversably permanent (powerstrips for posters work quite well; if removed they won't leave a trace of manipulation)
This is the most flexible solution that adapts most easily too almost any angle you like.
Also, from a related question:
Get a silicone non-slip cutting mat and cut it to size. It's flexible, customizable, and portable.
Cutting this so that it only slips under one (back-)side may work for you.
Or, like the answer there seemed to suggest: you might go for a permanent solution and attach self-adhesive rubber feet, buy them according to your desired angle.