Plastic boxes are readily available and cheap. Go to any marine store such as West Marine and you will have a mess of them from which to choose. If you for whatever reason still want to build one then I would suggest that get some 3/8" plywood, a pint of west system epoxy with hardner, fiberglass tape and some micro wood filler.
make a template of your battery
transfer the template to the plywood
cut out the wood
mix the epoxy and coat both sides of the plywood using a foam roller. 3 coats light sanding between coats.
using brads tack the box together
mix epoxy and wood filler to a consistency of peanut butter
make a fillet on all of the inside corners including the bottom using a tool similar to a tongue depressor
after making the fillet remove excess epoxy
when the epoxy has set but before it has hardened cover the outside edges with glass tape and wet it with epoxy using a squeege and getting all air bubbles out
when the outside tape has set cover all inside joints in a similar fashion.
when the epoxy has cured coat the box inside and out with two or three additional coats of epoxy using a foam roller.
If the box will be exposed to sunlight then it should be painted or coated with a uv protector.
presto you have a battery box
or drill some holes in the bottom and you have a live well for your boat.
Not real certain that after you read this you will still call me friend LOL
But I have used this process to make a darkroom sink and drain board. It works well if you have a special application and there just doesn't seem to be a ready made product that will do the job.
Keep the shiny side up.
WBCCI # 348
Past Region 3 President
Past President Tidewater Unit 111
Rick Bell in "Silverbell"