Do you really want to make your own when you can buy one for, well, not that much really; http://www.marinemegastore.com/category-Battery-Boxes-ECLBAT002.htm
If you really want to make your own, WBP ply with a GRP matt lining as JohnD says but it will most likely cost you a lot more.
I made a twin battery box with change over/isolation switch the same way but that was 25 years ago when such cheap alternatives weren’t available; make sure you provide adequate ventilation.
Not sure where you keep your boat but bear in mind that most river authorities now require a license & are regulated/inspected to minimum safety standards; you must meet these to get your license so whatever you do will need to comply with their minimum requirements.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local