That one has a slot rack attached to the structural member, but the slots can’t be cut into the structural member for a number of reasons, and I don’t think any design does that. The slot rack needs to be a separate component for a number of reasons. You already ran into the first problem- it’s too hard to cut. It has lots of other issues that have no real fix- it’s putting the location of the slats too high, there’s nothing to hold their angle.
I’d be happy to see what you’re imagining if you’d like to model it in fusion. I could even help machine it if you need. Otherwise, it might make sense to table discussion on the slats until things are running.
This needs to be addressed before cutting the slats because it’s a lot of material that will need to be replaced if it’s cut like that.
Considering that the material cost has been borne entirely by Eric and myself, I think that potentially needing to re-cut the slats is an acceptable risk 
It’s easily flexing through torsion from light pressure without even being in motion, so this isn’t working from a design perspective.
That is rather concerning, were you pressing on the rail, or the extrusion?
9:1 will be VERY slow unless the pinion is large.
It will be fairly slow, but as I mentioned we can change it if needed.
Precision does not come from the step-down ratio at all- and this belt drive is limited to somewhere around 1/8-1/4" cut precision overall, which is not very useful.
I’m afraid I’d have to disagree with you here, both on the precision gain from the step down, and the realizable precision from the belt.
But, perhaps it would be prudent to instead focus our attention on concrete ways we can improve the design. We agree that a rack and pinion would be better than a belt, would you be willing to provide those?