I have to admit that making motor mounts is very annoying, so I can appreciate what you've been doing (and it looks great)! I hadn't thought of using sticky-backed paper though (what I did was to drill the centre hole, clamp it to the motor in place, and then mark the drill holes).

When I come to fit the other ballscrew to my machine, I'm cheating, I'm afraid - the guy whose workshop it's built in gets a lot of parts laser cut, so the new ones (and also ones to replace the existing mounts, since they're being relocated) will be laser cut from 6mm steel (although the plates will be tapped and welded in place). It comes to just under £2 per plate.

irving - the second would be better, although I'm not sure I'd recess the plates either side of the motor mount (but would look nicer).