Alright, so it took me forever to locate my spares (found them ina box with a whole whack of other things that I can use right now; score). I did not want to rip the ones out of my Triumph as I didn't know how long I'd be and didn't want it out of commission.
So, the baseplate plan was a goiod idea but not very workable; the core of the pup actually hangs down out of the plastic cover by about 1/8th" so there would be a gap between the baseplate and the mounting ears. Damn. making a cutout for the guts wis doable, but I worry about a) my ability to do so as accurately as required, and b) whether that would not help structural integrity much on account of being so thin in places.
Thinking I can just make some sort of corner braces for the top and bottom mounting ears (the middle ones tend not to break on mine; they have more material around them and less stress generally). Won't be pretty, but if it does the job I'll be happy.