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 here but my printrbot has a fan so if you don't have a fan mounted on the front you may want to consider doing so when adding this bottom since you will be reducing airflow. The fan mount I used can be found here: