You're having all this fun by yourself?
Your 'autopsy' should tell you where the leak originated. Let us know. I'm just finishing my repair now. I had let the roof edges crack and water would pool in the front left corner. I had no water infiltration at the lights. BTW- if you do replace the lights make sure you get ones with both leads. The ones I just got on Ebay didn't have the ground lead and I had to solder some on.
Regarding your question: Why not just use 3/4" like the did at the factory? Factory made plywood will be way stronger than anything you can make in your driveway.
