Agree that FB is a great medium for one-off items and that it readily bonds to other materials, however it is labor intensive when it comes to quantity (even low quantity) production.
Have you considered making the corners out of a vacuum formed plastic like they use for coolers? Much cheaper than fiberglass and well suited for low volume production. One outfit that I know of is Sailing Specialties in Hollywood Maryland.
https://ssicustomplastics.com/custom-oem/
