When you get some time and have better weather or a garage to work in you can drill out the broken bolt and maybe use an easy-out to get the broken part out. Soak it for a few days before hand with your favorite rusty bolt spray. Take out the battery so you can get close to the area of work. Check the remaining part of the bolt you have and get a drill bit just a little smaller so that you might also be able to use a tap the same size as the bolt to clean out the threads on the battery tray. Take your time with the drill bit so you can stay in the middle of the broken bolt and not go off to the side of it and into the threaded area when drilling. If nothing else you might need to drill it out to the next larger size and make new threads with a tap the next size up and get a new bolt. Your original plastic hold down might be able to handle the next size larger bolt if you drill it out to that size. Once done with the drilling and tapping use some grease around and under the threaded hole and on the new bolt to keep it from rusting up and breaking off. Put on some new grease every time you take the bolt out of the tray and it will stay workable for the life of the truck. Or like you were thinking, get a new tray. Some of them don't remove to easy. But use grease on the bolt which ever way you go.