There should be no issues sealing fiberglass..
I don't think you will need to go 1" thick though.. fiberglass is stronger than wood.. if 3/4" wood works, you won't need the fiberglass to be that thick...
There is a bit of a discussion going on about how thick a glass enclosure needs to be, more specifically, what type of glass to use (matt, fabric, etc etc)..
You planning on making that yourself or having someone do it for you? And why fiberglass?
(1) Alpine CDA 7892 HU ($214)
(2) 12" Pioneer Premier 1241DVCs in sealed MDF box (gonna glass them eventually) ($69 each)
->removing.. ->(2) 10" Pioneer Premier 1041DVC's in sealed MDF box (gonna glass them eventually) ($59 each)
(2) Lanzar Vibe248 1200 watt 2 chnl amps ($119 each)
It's bump'n GOOD!
You can lead a horse to water but you can't make a moron understand...