Creating smaller baseplates from larger ones is a relatively simple task. With the proper tools, a modified one can be practically indistinguishable from an original. This is possible because of the small gap that is present between baseplates when joined together which makes it possible to remove material to match an original.
Router table with flush cut bit
Use bricks as a guide to score the baseplate. This places the score line precisely between two rows of studs.
To calibrate the fence, use an original baseplate.
Use bricks as a guide for the fence on a router table. The guides are important to compensate for the recessed material on the original edges of the baseplate.
Carefully route the scored edge.
The corners can be routed by hand or a template can be created for rapid transfer.