MPMD SD Extension Cover remix

I had difficulty getting the original design to lie flat on my printer. My remix reverses the connection between the two halves so that the brace fits over rather than under the main cover piece. This should do a better job of keeping the main cover from sticking up where the two pieces meet.

In order to do my remix, I recoded the pieces in OpenSCAD, based on the original dimensions from 95cruzer's TinkerCAD design.

Update #1: Version 2 adds a tongue and groove to the joint between the two pieces to prevent any vertical motion. This will allow the cover to function as a single piece, even if the imperfect flatness of the printer deck causes it to bow a bit.

Cleanup of the main cover piece is now much more critical to allow it to slide, albeit snugly, into the brace. Pay particular attention to removing any support residue.