I am trying to make this an upgrade for the Wilson and the Lulzbot with minimum changes.
I will raise the bottom rails up aprox 20mm with 4 new bottom corner brackets to replace the std wilson corner bracket for the main hoop of the frame. that way the Linear rails will bolt direct to the bottom rails.
I have already made a motor mount that will bolt to 2020 and I would change the belt adjuster to fit onto 2020.
The motor and adjuster ends of the liner rail will have I guess you would call feet to take up the 20mm or so gap between the 2020 and the bottom surface so the ends are not "swinging in air".
I am using the strength of the rails as the complete Y axis frame and just replacing the Printed end plates with 2 short lengths of 2020. This would be fine for 200x300 and possibly 300x300.
For big heavy bed or you want to make it super strong then instead of the "feet" you could run 2 2020 rails the length of the Y axis to tie the motor and adjuster endplates together.