I have three brews on my new Blichmann kettles, a 10 gallon BK and 15 gallon MT. It was obvious from the first brew that keeping the mash temp up was going to be an issue. The first one I had to add some of the sparge water to keep it up. The second one I added that silver bubble insulation which did very little. The third brew I added a thick blanket with a heating pad on top of the lid - that finally did the trick.
Being kind of OCD about things looking good, I started doing research. I found Canvas Keg Jackets online at a place called Rapids (http://rapidswholesale.com/beer-supplies-draft-beer-equipment/party-picnic-keg-pumps-and-coolers/keg-cooling-jackets-and-blankets/brown-canvas-keg-jacket.html
) for just under $50 delivered. It is very thick and heavy duty.
I was pleasantly surprised that the thing fit over the handles & sight tube of the MT! I'll have to cut a slit for the thermometer and valve, but that is minor. There is even an opening on top (for the tap) so I can run the cord from the heating pad out of that. Here it is: