In helping a local gym construct two lifting platforms, we found the classic plans described on Ironmind to be a great resource:
The primary modification we made was the narrowing of the centerpiece from 48" to 40". Our reason for doing so was to prolong the life of the centerpiece, as we predicted the amount of lifting --and lifts dropped from overhead -- would be pretty high. To compensate, we used two 4'x6' stall mats per platform, each cut into two 4'x28" pieces.
Overall, the platforms are great to lift on. If there is one thing I would change, I would prefer using a thicker matting, like the 'Super Mats' Dynamic Fitness/Eleiko sells. These are the same type of mats used in commercially made platforms, and are also the mats used in the lifting platforms at LBH.