Many commercial boats mount their seats with combination of attachment to the coaming via cheek plates, foam blocks under the seat and in some cases, fiberglass tape "wings" between the seat sides and the hull. If you have a VCP boat that you can look at, you'll see what I mean. This mounting system is very strong, but does not allow for any adjustments.

Another option is to just make a foam seat. In the long run, it's probably easier than trying to mount a fiberglass seat.