I just purchased and installed four end-to-end golf cart battery boxes from Batteries Northwest located in Salem, OR.
They were very friendly, responded to all my questions by phone and email, and I received the boxes 3 days after ordering by phone using UPS ground! Anyway, in my wood 36CL, these boxes just fit PERFECTLY between the stringers. They are built from tough 3/8-inch plastic and, with the lids on, hold my weight (210lbs) with no problem. They come with pre-made cable slots for up to 4/0 battery cables. I drilled some extra ventilation holes in the sides and installed four boxes in parallel, running from the second bilge inspection plate behind the forward bulkhead. This makes the footprint of the four boxes 22" wide by approx 32" long and 13" high; or about from the front engine mount to the front of the starter motor. The whole set-up looks really neat in the space and eliminated my rotting old fiberglass boxes and plywood standing platform. i'll probably fab some kind of wood or checkerplate standing platform over the boxes to secure them and to use as a step down into the engine room. Maybe something on a hinge so i can service the batteries, which are the flooded Trojan T-105 plus. These, by the way, just cost me $113 A PIECE at Battery Systems in San Francisco. I was upset about the price until I found out that this month they're going up to $150 a piece (copper, lead, fuel prices, weak dollar, thieving bastards, etc). Anyways, at about $50 per box, all-in, its not the cheapest way to go, but after contemplating building a box myself and the time, supplies, hassle to do it, I just bit the bullet and bought these. More time for more fruitful boat projects (too many to name!).