15mm ZMR unibody spacer 3mm arms with shortened standoffs

This is a fairly thick ZMR 250 spacer designed to be used as a unibody, with all components between the two bottom plates of the ZMR. In addition to fattening the spacer from 10 to 15mms, I shortened the arm mounts from four holes to two holes, which I think is a massive improvement. Here is a list of reasons why:
* More interior space for components
* Less weight
* Less print time
* Arms can be bolted directly to bottom frame in the inner two holes, increasing rigidity and further decreas