This is the floor, passenger side, of a 2005 Roadster. My 2016 plus 4 is the same.

Note for each runner there are three holes. The front one is right next to the crossmember. At some point, there was a crossmember somewhere towards the rear. Check your floor. Nothing is impossible, but measure the length of the runners and the distance between them.

[Linked Image]

DaveW
'05 Red Roadster S1
'16 Yellow (Not the only) Narrow AR GDI Plus 4