Printrbot simple 1405 bottom

I struggled with printing out the other bottom I found on here

http://www.thingiverse.com/thing:469267/#comments

that used retention clips, and found that the retention clips didn't really hold the bottom securely. I build this new bottom out from scratch.

This uses the standard m3 screws that printrbot uses. Just drop in the nuts, drill 4 small holes that line up with the screw holes, and mount in place. This should be plenty secure.

Note that I have a fair amount of ventilation