Explanation of the Prusa's thick sheet is here: reprap.org/wiki/Thick_Sheet
This design has a few improvements:
-Support for Prusa Iteration 2's single thick sheet design.
-There are holes for belt clamps, lm8uu bushings, and Prusa's heated bed.
-Holes in the middle of the thick sheet to reduce weight.
-The edges of the holes are rounded to prevent stress from concentrating in small areas.
The design could be improved by adding holes to mount something for opto endstops and slots for LM8UU holder zip ties.
Update: I adjusted position and size of the holes for the belt clamp (so the belt can be shorter), heated build plate (was a tight fit in original), and LM8UU holes (so no print area is lost).
Update 03/14: Corrected position of the belt clamp holes due to NikkeiRonin's suggestion. Added version for 3 lm8uu bushings like NikkeiRonin's thing.