Wow, how did I miss this? Previously searching for Tesla Model 3 cupholder inserts only returned results for collar/ring inserts (the ones that get stuck to your cup/bottle) or liners for the bottom of the cupholders. This Evannex insert is exactly what I've been looking for. It has the flaps to grip onto cups/bottles of various sizes vs. a fixed diameter ring. Also, you don't lose the use of the space connecting the two cupholders as you do with the ring or the spring-loaded design. That connecting gap is handy for my eyeglasses case or mobile phone when I don't need to carry a cup or bottle. Finally, since it is fully enclosed on the bottom and sides, spilled liquids won't get on the cupholder itself.
Thanks for the heads-up!