I cut mine out of a 1/8" G10 sheet. I'm not sure that 3D printed material would be strong enough. There is a pretty good air load on these when they close and also, when closed, the air cylinder has not reached its limit, so its putting all its force on the arm. That's good for keeping the door closed during flight, but it does put a lot of stress on the hinge. Perhaps if you backed up the 3D printed part with a think sheet of polyply (essentially G10) or a layer of carbon fiber.

Bob