I use your idea but guess what? I wedge it in the armrest (aka phone craddle).
99% of the time I am the only driver.
Once in a while, there are kids in the rea so it is very useful.
The key thing is use asmall piece of cradboard or paper to wedge it in the arm rest. See the pics.
HTH someone looking for a cheap solution for the REAR cup holder.