I've not had any problems with the firebox door , also. There's a leak in the upper left corner, but until that leak impacts the ability of the controller to keep a steady temp, which I don't foresee, I won't worry about it.
I'm still not sure what you're calling a gasket. Is it the rubber seal that fits around the outter edge of the hopper and firebox doors ?
On the hopper lid, and this is true of even the high dollar GF's, most people put a sheet of aluminum foil over the top. This helps the gasket seal and protects the lid from creosote buildup. But the LSS hopper cover eliminates that.
LSS gasket is just a felt gasket, its about 1" wide and its pretty thin. If you can work with stainless , you could make one very easily. I think there's a vid on LSS site showing the installation that gives ya good view.