---

I had the battery box screwed directly to the floor of my 15' with 4 #10 wood screws for several years. It was behind the seat (and offset to port) so it didn't get much pounding. I have since moved it forward under the console.

The wood diagram for the 13 shows a 1/2" x 4" x 12" piece of plywood right where you say it is. It runs parallel to the beam with the aft edge 7" forward of the point where the transom well meets the floor. Oh hell, here's the pic. (I bet Joe has it archived on the site somewhere too.)