That's pretty good, and you could always lap the top surface if needed.
An inexpensive solution is terrific. Not only would it allow more people to make one of these, but it could also be a bench mark to see what gains, if any, are gotten by using a more expensive system.
We've been focusing on 1:1, but for many people that would be overkill with large format. (For years I only enlarged 4x5" to fit on an 8x10" sheet of photo paper with 1" boarders.) Using 1:2 or less would lead to much more depth of field and require less tiles. In addition, most 50-65mm standard macros are optimized for less magnification than 1:1.