The default rostock-mini design mounts the print bed directly to the base of the printer and adjusts the leveling of the hot end nozzle via three adjustment screws on the carriages that home the printer. This may work, but I wanted to make my round glass print bed removable (mounted via binderclips). I figured that if I was going to add an under-hanging lip for clipping purposes, I was going to need some space below it anyways, so I decided to make it adjustable with 3 screws at the same time. I mounted the adjustment knobs under the platform so that I only reduce the build volume by 5-10 mm total. This is my current platform design:

The rectangular areas are where binder clips attach. The holes in the rounded tabs are for a hex bolt and the two rectangular slots are for pins from a plastic piece that keeps the bolt head from turning.

(This allows a knob to turn on the bolt as a hight adjuster.) A piece of adhesive cork acting as insulation sits between this plate (which will be cut from 1/8″ birch plywood) and the glass build plate. The glass plate provides the structural rigidity and provides for a nearly flat build platform.

If you want to replicate this, all my design files are on Thingiverse, #63414.