I used to build aluminum horse trailers,lots of doors on them,we had a custom extrusion that was in the shape of a Z,2 inch flange on the skin side,1 1/4 web for a door and a 5/8 leg for an inner flange to keep the water at bay.You could bend up that shape with your offset knife.We made double skinned doors to fit the openings,and weather stripped the inner flange.It really doesn't steal all that much space.