Rigidbot enclosure remix - heavy duty lower clip

I'm adapting http://www.thingiverse.com/thing:488135 for use on my Rigidbot Big and the laser safety acrylic I have is thicker than those clips are designed for.

Warning - i think version 1 will interfere with the Z axis limit switch tightening bolt. Version 2 is now available and should avoid that issue.

Changes:
Longer frame bar supports
Added support that clips over lower panels for more stability and to hold them in place
Accommodates up to 3mm thickness acrylic sheets