I am using the 20mm linear setup from VXB I love that stuff BUT!!!! the to Y rails you have will flex even at the 25mm size so what you need to do is support them vertically some way. I love the design but I am thinking that 2 flat bars sitting vertially and joined to the points at each end just below the y rails will give you more support and stop the flex you will get the beaings need to be the "c" style ones or open rather than full circle ones. this will stop the issue cold I think.
I can whip out a sketch for you and would direct toward my thread and the pics I post mostly the last one dealing with this flex issue.
Steele and aluminum first project
Let me know on the sketch