I have the battery under my console at the stern end.
You know m10 bolts that hold the console down, well on mine there were alloy stretcher plates across them. I replaced these with stainless strips 2"x1/16 thick and welded a 5mm dia stainless bar onto each one horizontally with a gap under to form a place to hook a strap. I then used a small stainless ratchet strap, ditched the hook end and ran a continuous loop through the eyes over the battery and both ends through the ratchet,
clickety click and its not going anywhere!
I dont know if a battery tray is worth worrying about, i find theres plenty of seawater and rainwater slopping around in the console to dilute any acid
havent smelled any chlorine yet