The stuff to use is like this:
http://www.ebay.com/itm/Ceramic-Fiber-Insulation-Blanket-for-Wood-Stoves-or-Inserts-By-the-Foot-x-1-/271166851971 I actually got mine from McMaster-Carr long ago, but this is a better deal with similar properties.
Rated to 2400°,
looks like fiberglass insulation (which is only rated to 1050-1200°) but it is made out of a much more heat resistant ceramic, rather than glass. It still has many of the properties of fiberglass (flexible, compressible, little fibers will break off and are somewhat itchy), but the insulating power is considerably higher. Cut pieces with a razor knife to fit the top and sides of your roaster, cut some sheet metal and bend it to make panels slightly larger than those dimensions, then tape the insulation onto the sheet metal with aluminum tape. Only takes a few minutes, and you wind up with custom panels that allow for quick placement and removal, and keep the insulation from getting messed up. I used to just set the insulation over the top of my grill and set a couple of weights on it to keep it in place, but I kept getting fiber fragments all over everything, and the solution was the removable panels. EDIT: the aluminum tape does lose it's stickiness over time and use, and needs to be replaced, pop rivets and washers are definitely the proper solution, I've been too lazy to do this so far, I just keep replacing the tape.
As has been said, not necessary (or even advisable) for commercial roasters, but for a BBQ roaster it is critical if you want to heat efficiently, save propane, and gain more control of your system.
