Here is the basic, still all-solid OpenSCAD model of the rail guide. The manufactured accuracy of these cheap and plenty hardware provides a baseline, to fine-tune to better accuracy. For example, unless I am able to maintain a 1 mm tolerance over a standard, factory-sliced 48" segment there is no need to look for "better" hardware, I need to work on my models. Then of course, if I can move this into a 0.1 mm range in a repeatable manner YOU can make a use of it.
Next is to add more concave trapezoids (called dovetails) to allow lock-down, attachment, leveling, and of course to lighten and strengthen the design. Currently this is at 32 grams, and takes 3.5 hours to build on Ender 3. I call this the "solid", as there is nothing left to "add". Next is to remove material.