Our 04 CC Allure has a battery bay with the open to the ground egg crate metal flooring. In the future I want to convert this bay to support a Lithium House Bank (No rush on that, as our X's 4 Lifeline L16's remain and should go another ~4-5 years

!).
I currently have 1 CAT 8D Maintenance Free Chassis battery, which is on it's way South. With the idea of making the Battery Bay more sealed off from the elements to support the more sensitive temperature protection of a future Lithium House bank, I'm going to shift the Chassis battery to AGM. I want to close want to close of the egg crate metal.
Looking for input on materials to use to do this:
-Marine plywood
-Hardie or Durock
-1/8" thick plastic
-Other?
I'm currently planning on X's 2 Lifeline Grp 31 Starting batteries, which will not fit in the same spot that the single 8D does. So will ned to raise the floor area of this section to be flush with the side rails, about 3/4" deep, and then mount two individual battery pans to this for the batteries to be strapped into. So was thinking the 1/8th thick plastic, with Marine plywood to shim up the balance of the way for the battery pans.
Until I do the modification to fully close off the battery bay basement the rest of the way, some moisture from rain travels will work it's way into the area. (Bay is just behind the passenger tag wheel on our coach, and the drive/tag even with a splash guard, do have mist/spray blowing around behind them while driving rain.) So thinking the Marine plywood might need to be replaced at after 4-5 years of moisture potentially getting onto it. (Which is not a big deal costs wise.) Suspect Hardie or Durock would survive getting wet longer, as they're designed for outdoor usage too. But then wondered about mold?
Suppose I cold get several nylon cutting boards, and stack them on the floor grate to the proper height - they'd never need replacing.
Anyways, hope the idea of what I'm trying to do is clear, and would welcome input and suggestions.
(Note: For the future install of Lithium House Bank, will use a combo of appropriate heating pads, and it looks like a pretty easy mod to splice into the heating bays heat source, to splice some heat to this bay too. Will sort that out at that time... Just trying to do the Chassis battery segment of the bay now, which is about 1/4 of the overall space.)
TIA, and best to all,
Smitty