Since my objective was to be able to go off and do something else during mash time, I roughly sketched up plans for a tea cozy for my pot and got my wife to sew it up for me. We then overstuffed it with polyester fiber insulation. I arbitrarily picked a wall thickness of 10 cm and double that for the top. The final result looks like a strange cross between the Michelin man and a toadstool.
Keeping with the better quality sleeping bag theme, she put some baffles inside the walls to keep the insulation from moving around. Cheaper sleeping bags are sewn right through to keep the insulation batting in place, but that loses a lot of heat. My wife had a lot of spare fabric lying around (fortunately not too garishly colored or patterned), as well as some of the fill. She estimates that it would cost about $100 to buy all the materials new. Using a cheaper insulation material such as cellulose from a building supplies store might bring that down a bit.
It works fantastically. My last mash lost only 1 degree C over 90 minutes. The only downside is that it's bulky, doesn't compress much, and I haven't got a clue where to store it. However as you can see from the last picture, it does have secondary uses.
This shows the beast facing off against its weather adversary. The opening at the front serves two purposes. The jacket is a bit of a tight fit over the pot, so the opening makes it easier to get on. It also allows me to leave my thermometer probe cord in place. The closure is a Velcro flap.
In action! The jacket is a bit longer than the pot is high, so you can only see the legs of my burner. The two webbing loops on the top of the jacket make it easier to lift it off of the pot.
"I don't care if it's your brew day. My nap has priority!"
Paul
( even as cat beds)
Still can't believe you guys can brew in those climates.