Purchased a cheap electric fan heater to use for camping but wanted to raise the front to better distribute the heat. I printed these in FLEX and they are easy to install. The BGCODE is for printing it in FLEX. Bit of a niche item though ;). 

How I did this? Simply used Prusaslicer (see attached file Mod Raiser): created a box of the size required. A second smaller box was added at the maximum height of the angle. Used a negative box (angled) to create the right offset angle. It would be easy to make changes to the Mod Raiser file for your purpose. Before slicing remove the second box. I like to keep it simple ;)