I had the same issue with my son's booster. The seatbelt receptacle ends up under the seat making it very hard to buckle for him. I moved his seat to the passenger side, where there are 2 receptacles (One for the outboard seat and one for the middle) and he can get at it a little better. Ultimately I ended up getting him a 3" rigid seatbelt extender and now he has no problem at all. I know that seatbelt extenders aren't recommended for use with booster seats, however the tesla buckles are so low that the 3" extender ends up right about were the seatbelt should be and doesn't change the belt path at all.
Good luck
This is the extender I got. 3" rigid.
Tesla Model S Seat Belt Extender