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Gldnby is on the money on this one. All this gasket stuff is bunk. I have four eggs and have never had a DFMT fall off!
You have to face the pivot screw in the top towards you, so that it does not swing open when you open the dome. Inertia from that swing is what makes it fall off. Now, you have the pivot bolt facing you and you have achieved your target temp. Turn the entire top, base and all, so that a line through the pivot bolt and the center bolt on the butterfly are parallel to the line from the egg handle to the hinge [the opening axis of the dome]. Now when you open the dome the DFMT will stay on and your settings will not change. Somewhere on here is a diagram of this, probably on GG's "Tips and Tricks".
I've had my Large Egg for close to 10 years and the DFMT has NEVER fallen off...because early on someone on this forum said to make sure the pivot screw is at about the 5:30 clock position as you're facing the Egg.
Forget the gasket stuff, this really works. I have three Eggs and the toppers have never dropped off.