They all have sloping ends because they are made in a mould and to get them out of the mould they need to have a chamfer. I won't go near straightening the plastic ones, it makes them too weak. The ally boxes can be machined straight, badaboom is the man to advise about that.
Also not all boxes are as bad, I have had a few Hammond boxes that have been near enough straight. I also had some very cheap boxes from Rapid electronics and they are actually straight!
Sent from somewhere inside a huge cloud!