Phone around in your are to find out what companies specialize in insulating piping. When they install a new hot water heating system or air conditioning system, the contract will often call for the piping to be insulated so as to avoid heat transfer where it's not needed or wanted. Those pipe insulating companies will buy their fiberglass pipe insulation from a local supplier, and that local supplier will also sell a product called Roxul.
Roxul comes in 2 foot by 4 foot panels in various thicknesses with 1 inch being the thinnest. It's a mineral wool matting that is rated at 1200 deg. F, which is far higher than you need. The companies that insulate pipe might also give you some ideas on how best to install it, but I think that if you just had some sheet metal bent into a dog-leg profile, then screw pieces of those to your boiler cover to hold the insulation in place.