I used the AxcessAbles 16U - https://www.amazon.com/gp/product/B07XWL296H/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1 There is a Samson one that is popular, but this is a good bit cheaper and seems to be identical. It is sturdy and assembles easily.

Came out pretty close to the render, which isn't always the case.

@Kstan Simit the magnet base may not be the best choice if I am lifting a heavy load or moving at high acceleration rates. However, if properly designed it could work even then. The only weak spot right now is shear. That can be mitigated by adding anti-slip bumpers adjacent the magnet contact points, or worst case having magnets not just on the top of the cart but overhanging an edge or corner. Ultimately the base will be made in a different material.

Made some progress on the custom pieces. The stepper controllers are all mounted on the DIN rail backplane. I accidentally assumed there were 6 of the larger DM542T + the travel rail when laying it out, instead of 5 + 1 smaller DM320T + the travel rail. I'll call it "future-proofing" :-). Also made the first prototype of the 1U cover from 22ga steel. It's very flimsy when the flat pattern is cut, but gets very stiff (more than enough for this use) once the flanges are bent in. I had to do some cleanup on every hole as the plasma undercut for some reason - likely I need to adjust the kerf for this material. Next step is paint!

@Chris Annin Thanks! I was going to put it on a 6U door similar in height to the plastic box from the BOM until I realized it might fit on a 1U. Should leave lots of accessible room for servicing.