It is a fairly easy fix if the framing iron is solid. Sheet metal is readily available precut to size in a sheet metal shop or any supply,Home Depot, etc., just cut with shears and rivot or bolt in. Leave a drain in there somewhere where water will not be trapped.
Plywood could be used or a plexyglass I would think also.
One note: Be sure to not raise the batteries or said another way, be sure to have clearance so your tray will go in and out without the battery terminals hitting or grounding out on the top of the enclosure....
I did not have to repair mine but put a thin layer of rubber in there to cushion and isolate my batteries...just a thought...lol..jem
The Silver Buffalo
Saturn with Blue OX towbar
WBCCI # 14067