3. Chisel the radius (post processing)
Ultimately, you can achieve a right angle on inside corners in post processing. To achieve the desired angle, corners can be chiseled by hand. This is the least efficient method but there is no alternative to achieve the same result.
4. Consider another method of fabrication
If none of these options work for you, consider another method of fabrication. For example laser or water jet cutting both have no problem cutting inside right angles. You can compare these services here.