Any discussion about battery installation would be incomplete without addressing securing the batteries into the vessel.
For this task I often prefer a set of heavy duty pad eyes and some canoe/kayak straps.. The other pad eye in this photo is for the house bank which has not yet been installed into the battery tray. Containment of acid is critical so that any metal tie downs don't get compromised by acid. For containment plastic battery boxes suffice.
Also important is that the battery box not be able to move fore, aft or side to side. In this installation it is entrapped by a 1.5" tall X 1.5" wide mahogany frame on the battery platform. There are many ways to secure your batteries, just be sure you do.
A good way to think about installing your batteries is to mentally visualize your boat in a knock down. So, take a moment to picture your vessel in a knock down........
Now, where are YOUR batteries...? $hit happens so please install your batteries safely. A little spilled acid, if you have flooded batteries, is better than a 65 pound cannon ball flying through your cabin.
I generally dislike the cheap battery hold down straps that come with plastic battery boxes. The AGM photo before this one is a prime example of sub par hold down straps, neither of them was still operable... These cheap straps, with their plastic buckles, are marginal to useless at best.
EMS, LL Bean or Northwest River Supplies (NRS) sell the type of canoe/kayak straps I use. They work extremely well and are very, very strong..