I searched TMC but could find no posts about placing a Fastrak transponder (for bridge and road tolls) in the Model X frunk to see if it would work. I'm going to try it. This photo shows the small cavity on the forward wall of the Model X frunk. Seems like a good place for a transponder, mounted with some industrial strength Velcro. I don't think there is much metal in front of that cavity, mostly plastic I assume.