MOTM

All - new to MOTM, I'm designing a case for the modules I'll be
building.  I'll build the case out of wood - and I'm planning to screw
the moduldes directly into wood rails - top and bottom.  In your
experience, what should the optimal spacing between the rails be? 
Based on the face plates of the module kits I've recieved, I'm
thinking 7-7/8" should do the trick.  Sound right?  Thanks much.  Bill