Get a thick aluminum plate you can attach to the Y axis of your gantry or bridge, and screw a rail on each side, making sure they are square to each other. Run the ball screw straight down the middle, fixed at one end in a couple of angular contact bearings, with a bearing on the other end that lets it float a little. Make a motor plate with holes corresponding to the holes in your motor's faceplate and center it on the fixed end of the screw, then attach it to the plate with the rails on it. Get another aluminum plate and drill it for the spindle mount you're using, and also on the sides for the trucks that ride on your rails, spaced so that they will give good support without restricting the range of travel too much, and in the middle for the carrier that holds your ball screw nut. Put it all together and you've got a Z axis.