I had mine apart there, and as I recall I just used automotive gasket paper. There is high temp gasket material available but most places probably don't stock it. Edit: here's another thought, when I did home heating service years ago, because of the large variety of furnaces & boilers, we rarely had the correct gaskets available so we just used furnace
cement on covers, or joints, or wherever something was put together. If you know anyone that does that type service work, get an oil burner flange gasket and cut what you need out of it. Otherwise I think you'll be OK with the automotive gasket material. I always keep various thicknesses on hand. Another thought, several years ago I needed to make a manifold gasket for my old International truck, and my local NAPA store special ordered some high temp gasket material for me, but I never used it because I found an original gasket. I only used our furnace a couple times after making the gasket, then removed the furnace, so don't have long term results.