If you are going to use cantilevered wheels with such a system I would suggest that you weld your dead axels to a large metal plate and then bolt this plate to the fiberglass. Otherwise you will be concentrating a large force on a piece of material that is only 1/8" thick
I would be worried about an axel tearing out.
Another obvious solution to this problem would be to use a thin sheet of fiberglass to support the outer side of each axel. This would also give you a place to mount the bumpers.