Its a bit of a 'piece of string' question as it depends what good metal there is to weld to. Most import buses have a rotten tray with perfectly decent surrounding metal so its a fairly easy job. Any garage or welder will be able to do it - the only issue is how they do it - from unpicking the original tray welds and replacing the tray correctly through to tacking in a bit of flat of metal cut to size ...
As some advice, I've just fixed a tray on my brothers bus as he had a bit of metal in there that wasn't welded in, just put in place! Anyway, I took that out, trimmed away the original rotten tray leaving a lip around the edge. I then took the new battery tray panel and cut away the folded edge so it fitted flat and snugly onto the old lip and then used panel bond to fix into place. Then seam sealed above and below resulting in a solid correct panel held in place with no welding, and importantly, no welding damage to the external paint of the bus. Cost was just the battery tray repair panel, a tube of panel seal and some primer and paint. Its a job anyone could do on their drive too.