Jeff - I tried checking out the image - sorry 403 - no access rights on the server. If you post maybe I could get you better commnmts - but so far:
Yes you can use fiberglass as a mold for thermoforming. I have done it - and it works. I did need to reinforce the insides of my mold - for fear it would collapse iunder the pressure.
I use Spray Cooking oil just to make sure it would release from the plastic. I have seen that others have tried/used baby powder, felt, etc.
To get your mold really smooth - yes, gel coat will do that - but make sure it is compatible with your "master". First wax the inside of your master - several times - polish between. Make sure you miss nothing. For good measure, spray in some PVA - you can get at a marine store - smal quantity. (Or go to McMaster and get some spray wax. Use something designed for the resin - or you might pooch the resin.
Lay it all up in layers - take your time - don't rush and make to thick a layup. , when cured - pop it - and enjoy holding you new master whle sipping a cool one.
There - that is Just a start. Sure others have ideas too - or ask for more help