I have seen these supports published with little explanation as to their purpose. Some call them Z support but they will have no effect on Z movement. X support is a common name, I used it myself, but it does not effect X movement. It is actually the Y axis that is stabilized. A cantilever beam is not the most stable structure and the free end can move around. If you feel the end of the X axis beam during printing, you will feel it. So we come to the X axis support..
I started with the X axis gantry by 70-77. It has dual top rods that will make the support very stiff and provides mounts for other accessories. And it has the ridiculous over-engineering that I like. I used about half the parts and the only change is adding bosses for set screws to secure the rods.
To mount the gantry_d slide, I drilled and taped the X-Axis end cap for M3 cap screws. I used hot glue to mount the bearing.
For the base of the vertical support rod, I went a different direction. I liked the adjustability of 1cheeseball1 mount and modified it to use the 8mm rods that 70-77's design calls for. I incorporated the mount into a side panel that included the no-drill bed rewire feature. A bonus of the mixture of these 2 designs is that if you get 8mm x 300mm rods, you do not have to cut them. If you want a base that mounts on top of your existing side plate instead of replacing the plate, just use the base in 1cheeseball1's original design.
side panel b is large but will print on a Select Mini if you do not use any attachment.
3 8mm x 300mm linear rail rod
1 LM8LUU Linear Bushing
3 M3 x 10 cap screw to lock Y axis adjustment and mount gantry_d slider
4 set screws, I used #6 Imperial
4 M3 x 20 cap screws
4 M3 nuts
8 M3 flat washers