The mounting bars were not cut to exact size. Now, I would like to fix this problem with a borrowed hand saw... it's a hard job indeed, but all I need is patience.
The left side was already finished, the right side is being sawed... it takes a lot of effort, 100 "slides" translate to around 1.5mm at best! 6061 is just strong.
Cutting off the excessive mounting block makes the whole thing looked a little bit better, and it's also more "portable" in the sense that less strange parts here and there.