I don't really have a close up of the corners pre-paint. Now the corners are sharp 90 dregrees. I welded the skins on to the 1-1/2" x 1-1/2" square tubes directly. Nothing in between (glue wise). The point on the outside of the box were the two skins met top/side came out pretty clean after grinding, but to insure there wouldn't be any cracks, I used JB Weld for that. After priming and painting I use silicone sealant and basically covered the entire inside of the box making it water proof you could say.
If you don't mind me asking, how much do you think you spent on this project for materials and how much time did you spend? I am looking at buying a M101 and modifying or building from scratch ... looking at the end result of your project, building is definitely my preference. I just need to justify the tool expenses with my wife :smilies27