I also have a full size spare, but it is 'nailed' down, using the Honda supplied holding down bolt.
This screws into an existing tapped hole under the carpet, just make a small hole in the carpet to pass the bolt through.
One of my hobbies is model engineering and some years ago, I used to carry a model steam locomotive, weighing around 100kg, in the car. This was securely bolted down to a custom built tubular steel carrying rig, which itself was bolted down to the car. Can you imagine the mayhem that would have caused, had it broken loose, in a crash, or even just an emergency stop.
It never ceased to amaze me how some of my colleagues transported their models in the car, aka wedged in with bits of wood and old cushions. In my opinion it was just asking for trouble.
So, spend the £7 of so and get the right tool for the job.