FoolontheHill, these are the measurements from my 2nd gen.

These are inside rail to rail
front left corner to front right corner - 4' 7 3/4"
front left corner to rear left corner - 5' 11 7/8"
front right corner to rear right corner - 5' 11 3/4"
rear left corner to rear right corner - 4' 7 7/8"

Also keep in mind on a second gen the rear corners of the inside rail are rounded and not a 90* like the front inside rail corners