Thanks for taking the time to do the drawing, the closing plate idea was going to be my strengthening measure if the channel flexed to much. Using box section as a rail but rotated had occured to me, but I was put off by the radious on the corner of steel box section, Ally has a nice square corner but wouldnt last long with the bearing running on it, I'll try running a bearing along the edge of some steel box and see how the V matches the radius.
No probs with the drawing, probably took me less time than trying to explain in words.

Well at least you should have have enough options now.......looking forward to seeing some pic's

It may not be much use but Pacific Bearings have a 'square' bearing using hi tech plastic as the bearing surfaces, each side can be adjusted for clearance .. but I think these may have to run on steel box section ...
Not sure I would bother personally, looks like theres alot of surface contact so high friction and being fully enclosed means the rail cant be supported. I would also guess that the rail would have to be solid as box sections often have slightly concave faces.

Still i'm sure someone will prove me wrong :heehee: