FF, 2'-3" wide, 1'-4" front to back, continuously supported on sides and front, the john will be in the middle and its seat is roughly centered over the front edge of the base. The 'glass is 3/8" thick and will be reinforced at the bolt positions of the john. I will add a little something - 'glass blocks - between the reinforcements and the hull.
McMaster-Carr This table shows the stuff; interesting the wide range given for the properties!
The original base was 1/2" melamine-faced plywood with two screws holding it down.