One of my pending projects is to buy some Reflectex insulation and sew a "hat" to put over my mash tun, much like you are doing with the bag above, but with actual insulation material. There was a thread a few years ago here on the forum of a guy that did one. I have a single-tier BIAB stand, so that would make it so I never have to move the tun (right now I put it on the ground and wrap it in moving blankets). The thread reported that this guy lost 1.2 degrees in 60 minutes in 40F weather with this Reflectex hat.
It would be like a $20.00 project, I just don't exactly know how to sew so I've been putting it off!