There is some issues with those channels, they are too deep at sides (should be 15 mm, but they are 19-20 mm) and the bottom (sill) is not straight in any direction. I have to buy a second can of aluminium bondo.I see you have the weatherstrip channel on your 1/2 tub. Fabien builds them like that now but mine was an early model so I don’t have it. I could weld it in but I feel that it would be weakening the corners where the outside sheet metal meets the corner. So I have opted not to cut and go with the overlap like Teseven recommends
The tub was also a little bit too long too at beginning, HT sides didn’t went over the edge.