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Not to sway you in a different direction, but just showing another option available for Model 3 / Y seatbacks: Amazon.com
I've used these a couple times already with my kids' iPads on some road trips and they worked out great. The main reasons why I went with these instead of a roof-mounted bracket was because my roof is tinted and I didn't want to put any suction cups on it (and didn't want anything hanging that would be in my review view), and also because I was able to use a coupon code from a Tesla FB group to get them for $12 each last summer.