Prodded into action by Edward's announcement of the next version of Shapeoko, I dug up some plans that I had set aside last year, when I was still searching for a reliable extruder. It's provisionally called MegaRail (an incredibly predictable and boring name). I drew it last night and uploaded it to GitHub
MegaRail (ugh) is a linear motion profile that's twice as tall and twice as wide as MakerSlide. It forgoes the T-slots for a significant increase in torsional rigidity (or cost and weight savings, depending on how you look at it).
The essential dimensions (Vs, mounting holes) are the same as Misumi HFS5-4080 extrusion with OpenRail bolted on it in all four corners (only stiffer, lighter and cheaper). The clearances are different, though, and the Vs themselves are wider, supporting not only the standard Delrin V-wheel and the steel RM1 dual-row V-groove bearing, but also the larger RM2 wheel (and a possible upgraded plastic V-wheel). Because MegaRail (sigh) has Vs in all four corners, "dual-X" carriages can run on a single rail. This way, at a small additional expense, most of the off-axis load on the V-wheels (weakest mode) is converted to radial load (strongest mode).
The wall and core sizes are preliminary. The top and bottom faces bulge mostly to help prevent damage to the surfaces of the Vs during manufacturing and transport. Ideally, the core holes would have a star-shaped profile that permits standard screws to form their own threads, but I have to do some research on that.