For Level 3, the corners of the plastic ammo belt shall be trimmed at a 45 degree angle that meets the outer edge of the cloth belt.
Just trim the ends slightly so the corners meet with the fabric belt, correct position for drop boxes, yes different makes are differently sized, not an issue.
Also take note of the first picture posted, notice the rivets on the plastic belt ends aren't right in the middle of that area so you don't have to be mm perfect there is no measurement to abide by
Belt button cover reference
Only raised area is the circle
One thing to also take note of when fitting the belt is once worn it can ride up on the back because of the angle of the kidney, which makes the sides sit little bit of an angle, this can drop the front of the plastic belt.