I've never done a mini tub, but I wouldn't use fiberglass. If your not welding it in maybe panel bond like 3m 8015 panel bond adhesive. I've been told that panel bond is actually stronger than welding.
You would have to clamp or drill screw it with the panel bond and let set over night. Just make sure it's where it needs to be cause once it's dry you'll have to cut out if it's wrong!
Someone else might chime in and give some better advice, cause I've never done a mini-tub, but I think panel bond would work.