water activated fiberglass casting material? Is that something from the medical industry (bet that's not cheap unless it's a job related subsidy).

You can easily mold cradles directly to you hull by turning the boat over, covering with plastic sheeting (to protect the hull), laying on carpet, then laying on fiberglass and resin. If you want to get really fancy, you can form up some clay or similar to support the carpet to form a lip shape around the edges.