Thats generous, thanks Westend. The trojan 1275 is wider than any battery tray I found online, and I assume larger than most battery trays in vehicles found in junkyards, but modifications could be made.
I'm thinking the galvanized angle iron with the holes in it for attaching garage doors would make up a nice ventilated perimeter as well as give attachments for retaining hardware and easy attachment to the sheet metal walls in the area.
The location is a bit unsquare on the 2 sides and 2 sides are floating with a support underneath the middle, so its gonna be a smidge more involved and custom than a lipped 13.5 x 8 inch tray. Incorporating A cold air intake/ ventilation is going to be even more involved.
I'll have some time to get this ready, what I do not have is money to throw at it at the moment, or even for the new battery, so My AGM is just gonna pull double duty when I am not plugged in and the Meanwell is not holding it at 13.6v.
I got the first layer of paint in the area today and buttoned it back up so I can drive around in the meantime. My poor horns are located under the battery tray. They were growing cauliflower from some areas and broccoli from the electrical contacts, but still work.