I would rather convert the printer to direct drive first. There are various variations which are not expensive. I went with a cheap BMG extruder clone.

I'm going to add, that if you cannot print the gasket, maybe you can print a jig that will make the gasket cutting more accurate. I did something similar with a carpet cleaner where the waste tank gasket was leaking. I printed a jig to allow a hobby knife to trace an outline and not wander.

I have been rather successful with Overture's High Speed TPU. It prints really well with my Bowden tube setup and it has a semi-rigid outer shell that goes away during extrusion. It sounds like exactly what you are looking for.

I'm at this point not sure how to do it correctly, so I'm giving an individual upvote to every of your comments and leave a separate reply here. Thanks for all of your answer, which are very helpful. I'll give it a try with the materials you've proposed and keep you posted here about the results. Great first impression of this community! 👍

Just for a gasket you can probably turn off retraction and print most any tpu if you go slowly. Depending on the shape and thickness it might be stringy but it can probably be trimmed of burned off. Sure direct drive would be better, but for just one flat print that doesn't need to be pretty you probably are fine.

You can print TPU using a Bowden setup, though you will have to slow your print down speed and tweak with your retraction settings too. 95A should work, softer TPU may work though I can't really guarantee that

It's easier to print flexibles with a direct drive setup. There are some printable direct drive mods for the ender 3 which others have had success with, may be something to look into!

I have a Bowden printer (Prusa Mini) and regularly print flexible filament. The key is going very (almost painfully) slow. I’ve gotten excellent results.

I used Fillementum Flexfill TPU in both 92A and 98A varieties.

Keep retraction low and just give it a shot with quality filament! Overture High Speed was referenced earlier here and is a great call.

I have a franken-CR10 V2 that is far from stock but still bowden, and print everything from TPU to ASA to CF Nylon with zero issues. It really comes down to tuning. Direct drive will always have less room to flex of course, but you can definitely get things working with your setup if you take enough time to tweak.

Good luck, and share results here on the community!