Design files have been released. See the "Files" section of this project.

Do you think there could be a variation to improve accessibility for those of us purchasing boards from American hardware stores like Lowe's / Home Depot? Specifically 1/8" boards? Is that wiggle room between 3mm and 1/8" inch going to be within tolerance?

I decided to use 3mm thick plywood for OpenMotion because 1/8" thick is increasing becoming harder to find online. I have found vendors on Amazon to be the cheapest way to source 3mm baltic birch plywood. They even sell it in 300x600mm sheets just for laser cutting. 

If there is enough interest in this project, I would think about creating 1/8" thick versions. I would not recommend using 1/8" thick plywood for a 3mm thick plywood design. Likely to have issues with the thicker 1/8" thick plywood for tabs and slots on a 3mm design.

Are there any CAD files or at least drawings? So far I only see a few pictures. Or am I missing a link to some repository?

Nothing has been released yet. My current plan is to release step and dxf files of each stage when I have completed the second design iteration for each. Happy to release anything earlier upon request.

That's something that would also look great in my Boxes.py project (https://hackaday.io/project/10649-boxespy). Although I understand if you don't want to switch from CAD to Python.

Here a few lessons I learned while laser cutting plywood for mechanical things:

Use fully closed boxes as much as possible. Cutting big holes into the sides is fine but any open side will be a large weak spot. This is especially true if you have dynamic loads or diagonal or twisting forces.

Mixing wood and steel is tricky due to the large difference in surface hardness. Ball bearings typically work well as they increases the surface area from the steel bolt inside to the wood on the outside. Rolling on wood is problematic. Ball bearing balls will get pressed into the surface on all but lightest loads and cause permanent damage to the rolling surface. Full ball bearings are much better as they have larger radii and allow full width of contact. But if they are rolling on the edge of the wood they have similar issues as the bearing area is just too small.

As it looks like this is what you are doing with the V groove bearings I expect some issues there. Bumps will create dents in the edges so the guiding will deteriorate quickly. Having smooth ball bearing roll on perpendicular surfaces is much more robust - no matter whether is it the surface of the plywood or an cut edge.

Thanks for the comments. All great points. 

I admire what you have been able to do with the Boxes.py project. A great project!

If interest in this project grows I will certainly think about parametrized designs.

When bolting to wood, I use washers at the wood to distribute the load and lock washers to correct for any deformation of the wood over time.

I have had good results with these v-bearings riding on the edge of 3mm plywood with my other cnc machines. I think success has come from using enough v-bearings to distribute the load on the rails and radially preloading the bearings to account for the deformation of the wood over time. Agree that there are much more robust options, but v-bearings are cheap and don't take up much volume. Will definitely look into it if this project gathers enough interest.