My kiddo swears by those silicone covers, actually. The wax just seemed to disappear by lunchtime, but the silicone ones hung on through school snacks and all. They’re a bit fiddly at first, but once you get the hang of squishing them on, they stay put way better—at least for us. Still have to fish out bread bits with a toothpick sometimes... but the mouth sores definitely healed up faster. Hang in there—it does get easier after a few weeks.
Silicone covers are decent, but I actually found the wax easier to shape around the brackets—at least at first. That said, yeah, it’s a pain when it just melts away with hot food. I always kept a little kit in my backpack. Ibuprofen helped a ton for those first few days, too… the pressure aches are no joke.