Fiberglass Boxed for Seats DIY ?

[abl1111]

One of the guys had a pro make a pair of really nice fiberglass box seats to mount pedestals onto. I have a few questions as I've never manufactured from scratch - just lots of repair and resto. stuff.

It looks like a design was built out of a composite material and then glassed over ( as opposed to making a mold )

What material can be used to build with ? How do you make nice curved radius' on the corners ?

How and with what would you insulate for a fish box ?

Or, is it easier to make a mold, mold release, clay radius' and go that route ?