I've got some duct insulation that is fiberglas with an aluminum face on the outside. I'm going to cut to fit a wrap to go around the outside of the kettle, and keep it in place using wire from a coil. It won't be perfect, but it'll be good.
The nice thing about the fiberglas is that it's not going to potentially melt from propane flame like the reflectix would melt.
I'm actually less concerned about this for the boil--though it would help with that for sure--than I am for mashing in my kettle using BIAB.