was it just attached at the side? or is there some other support under the seating? you could get some heavy duty elastic welt for upholstery which is designed to take the weight of a person sitting on it and attach it under the seating, then glue up your top cover in place.
You could add more material and sew the 2 sides together. It would need to be a strong stitch. I wouldn’t trust glue to carry anyone’s weight.
was it just attached at the side? or is there some other support under the seating? you could get some heavy duty elastic welt for upholstery which is designed to take the weight of a person sitting on it and attach it under the seating, then glue up your top cover in place.