This very 'old' design has been enhanced to provide some support for the clip. The difference is a rounded cutout on the 'outside' of where the panels come together. It provides some stiffening to help prevent any flexing the panels might want to do at that point.

The basic function of the clip is to bring panels together, without using any screws. The panels found on MakerBot and Ultimaker builds are typical of a 't-slot' screw and nut design.

I have used this design to reconstruct my CupCake as documented here: http://www.thingiverse.com/thing:5219

1) Print a few of these out
2) Snap them into places where you'd normally use the screw/nut combo
3) Rejoice!

This clip is not strictly a replacement for a single screw as it requires a different configuration of the panels (no extended tabs). So, realistically, you need to replace all the screws, and make sure your panels have no tabs.

This particular design is helpful for the bottom panel since it holds the power supply, it is heavy, and can use some support.

There are other clips in the OpenScad file if you care to play around.