Hi Dave,
I like your idea and have been thinking kinda along the same lines for my boat however I want a live well that sits behind the leaning post.
Per your description the back and bottom of the finished live well does not need to be finished because they will not be seen. And I guess the inside of it does not need to be a smooth finished surface as well? So it sounds like you have a good plan. I've only worked with epoxy so I do not know how long a working time you have with polyester. You can use a slow cure product to give you more working time to get the wrap around the plywood. It may get a little messy if you start too early in the wrap process if the epoxy or polyester has not kicked. I worry a little about buckeling of the material in places in between the two pieces of plywood. If that happens you may need to add capital C piece of plywood to support the outside middle of the mold. You can use very small finish nails (the ones with no head)to hold the piece of fiberglass to the middle piece of plywood. If done carefully you will not need to putty over them. I've used formica and hard board for my molds and I really like the hard board as it is cheap and easy to work with. It comes in two sizes at Home Depot... I think 1/8 and 1/4 inch and has a very smooth surface. As far as how long it will take to make the throw away mold? Hard to say but i'm thinking no more then 3-4 hours max if you have all your ducks in a row. I'm thinking that for my boat I will buy a Kodiak live well and just build a mold for a cover that goes around it as part of the leaning post/live well combo. I dont trust my ability as a plumber so I prefer to buy a tank with all the fittings attached already. Below is a pic of the stern of my old boat. The middle hatch is that of a Kodiak tank enclosed in the stern of the boat. Hope some of this helps.
Strick