A suggestion that I have:
I myself use a keggle for BIAB and I mostly do 3 gallon batches with it do to some space restrictions in terms of beer storage.
I had an issue with the temperature drop as well during my first round. So I looked around at various methods to insulate my keggle while keeping it on the burner. I saw things like mineral wool coming up a lot, do to its high fire resistivity. Ultimately, I settled on a hot water heater blanket much like this:
http://www.homedepot.com/p/UltraTou...ot-Water-Heater-Blanket-60301-48752/202710039
Some of the perks include that it is not fiber glass, it has a high R value at 6.7, and it is Class A Fire Retardant. It is also very cheep. I bought this, fitted it to my tank, and then tapped the ever living crap out of it with some high temperature foil tape. I made sure that none of the insulation is in direct contact with the burner flames. I will be able to test it fully this weekend when I go to brew again.
Id say the only real downside to this is that I made it a more permanent solution. Im not sure if I will have to worry about mold at any point, but I keep the outside of my pot dry anyway and take care not to spill liquids on it.
Anyway, I can give you an update as to how it turns out if you are interested. It is really easy to do! Its also cheaper and easier to work with than mineral wool.