i had intended to use 20 mm rails on the z axis but after reading the various posts of your good self I am now going to use 15mm rails and guides to suit so i think a redesign is in order.
the rear plate should be shorter alright i just left it long enough to house the motor with a body of 100mm long and a plate to hold it.
i suppose the travel of the z axis could be longer but i went with that travel because of all the discussions about the length of the lever created by the extension. i suppose it's another trade off , keeping it to a minimum while having enough scope to machine to a reasonable depth. i'll get cracking at it again tomorrow, my shutters are starting to descend !