I haven't finished mine yet but I just made a wood box. you gotta work it a little but if you do it right it'll bolt into the old air box holes.

 

Best solution would be to get some cardboard, and make a mock up of the battery tray, then go to a metal shop ask for some scrap metal (like 12"x16" or so of 1/8") and trace the pattern out on to the steel. Cut it out or just hand it over to the shop with a couple 6 packs of artisan beer and I'll bet you you'll have a battery tray in no time. Paint it up for rust protection, and have the shop weld it in or bolt it up.