This was my solution for carrying my mountain bike. I cut an spare loft board to fit the curve in the boot sill then screwed a suitable piece of tube to it using a couple of pipe clips to give it height off of the board.
Then I simply drop the saddle & take out the front wheel @ the bike fits in the gap where the middle seat back drops with the pedals against the backs of the other seats. The square hole is to allow me to operate the thru axle to secure the forks. It helps that my bike's a small, anything longer & I have to drop the drivers side seat back to make room for the crankset but either way it works well.
I have to say though that life is a lot easier since I took the tow bar plunge & bought a proper bike rack!