It uses 4 different printed parts, a 624ZZ bearing and springs to stabilize the platform against the inside of the frame. It is held with 2 M4 screws and 4 M3 washers. The stabilizer grips onto the platform snuggly. A part of me thinks the spring becomes too tense and I may modify it to be less tense in future versions. The springs that I use are the same kind that are used on the platform.

There is an issue where you can not use the whole 6" height of the printer because of a rivet  towards the bottom of the frame where the platform stabilizer crashes into. I can only use about 4" currently before I can find a work around. MY SD4 is disassembled at this point and I'll need time to reassemble it to working state before I can make one for that machine. If there is any interest in it, I will make it for the SD3/4/Apprentice as well.