how thick is the fiberglass mat that you are using? how thick would you say you are right now? 1/2"? 3/4"? 1"?
how much resin did you pour on the fleice and let kick before putting the mat on?
as long as you don't have any air pockets you should be fine.
if in doubt, put another layer on.
also, do you think you could stand on the box without it flexing? if so then I would say you are good to go!