We had an ice storm yesterday, and I realized that I do, in fact, have smooth rods. They're just tied up in my acrylic framed printer.
I actually have a fair amount of them, so I was able to work off of this design:
And thus, the product of yesterday:
Currently, it seems to have a build area of ~180x180mm, which is horribly offset (the orange square). The constraint is that moving the smooth rails outwards causes the pulleys for the drive line to collide with the frame. Fixing that is easy, but fixing it in a way that doesn't add a huge amount of volume (and print time) to the corner pieces is still elusive for me.
I'm discovering I kinda like doing CAD stuff so long as I'm using OpenSCAD. FreeCAD is easy to use, but I never learned to use it in a way that didn't screw you over if you needed to go back and change something you did at the beginning.