Moeller makes several plastic below deck tanks that might do what you want.
What gallon tank are you looking for?
Not knowing what the boat has, I would expect for it to be a 34 gallon (#032534) or 55 gallon (#032555) belly tank.
http://www.moellermarine.com/afterma...rmanent_tanks/
Problem is, I've not been able to find a distributor for these numbers other than Boaters World and a couple of other people.
Look at the dimensions and see if these will work, then we'll see if there are alternatives.
Bad comes to worst you got the people RABBIT said or
http://www.rdsaluminum.com/marine-stock-below.html