ZDoom

I had an idea to make an upright coffin, basically this shape but inverted (filled in instead of empty):



I tried and tried and tried to do something with 3D floors and slopes, but I couldn't get the result I wanted, the slopes and the 3D floor didn't interact the way I expected. I looked at some tutorials to make 3D slopes, but they cover basic slopes and what I want is very complex, so in the end, I just gave up on it. I'm wondering if what I'm trying to do is even possible...

Yes, it is possible. Learning requires patience and ... unfortunately a *lot* of persistence.

In your case, have you tried aligning the coffin sides exactly to the same lines as the 3D floor sectors?

Yes, I made sure the dummy sector is parallel to the sector in the level, so that all the lines align with themselves.

EDIT: I FIGURED IT OUT! It was just a matter of separating the sector into parts and giving them different tags.