I talked to Fuji late yesterday. I was told that the rubber piece on the X-T3 is indeed a channel cover and is tethered to the battery chamber. It can be pulled up though doing so is not nearly as easy as it should be. I was just able to do it with a very fine-tipped screwdriver. Their engineering in this case is less than stellar.
A couple of Osprey outtakes in DE. 150-600
Wonders of taking multiple photos........high speed poop......and a vulture photo bomb. Didn't see either until I uploaded at home. LOL