I wish I had a sample of the original, no matter what condition. Then I could create a correct replica. Even a photograph would help. Failing that, I'll do as Benicia suggests and trace around the handle housing. I'll probably do a 2D Cad drawing of the result so at least my version of this gasket will be on file to share. I imagine there will be a lip (recess) on the original around the inside of the handle casing. If an original one comes to light I'll measure it and then perhaps one of you kind 3D experts can create a file for future use.
I have a spare door handle so I had a look this morning;
The gasket is a thin rubber part of 2.5 mm.
I will check with a round rubber thread if it's works and I will take all dimensions for a 3D file.
This is a sketch of my suggestion for a rubber door handle gasket. If any of you 3D experts feel like using this to create a file for a 3D printer then charge on - send it to me and I'll give it a go at the local print shop Who knows, it may become a welcome addition to the growing collection of friendly forum services........
Not visible? I had assumed they would be similar in detail to those on my spider (see below). They would look much better if you could see the black rubber gasket behind. Of course, they'd be easy to make if they were simply cut from a rubber sheet. 3D printing wouldn't be necessary - just a waste of time.