Battery hold-down
The factory battery space in our model was molded out for a pair of group 24 batteries each in a plastic box, but it had room for a more substantial plastic box for two golf cart batteries.
I used a piece of line going over the top of the lid, to an eye strap on each side secured with #12 screws deep into the frp lay up. For access there is a bronze snap shackle on one end.
Different location than your boat, under the bottom step of the engine cover, but perhaps the idea is worth something.
There are probably several good solutions to this, but one thing to remember is that once in a great while any sailboat will experience a knockdown and whatever we do must function at 90 degrees, or close to...
I believe that battery boxes come in various sizes. I found ours at a good-sized battery vendor in my city.
Plan B would be to mold one out in cloth and epoxy, to exactly fit your batteries and your boat's location.