Reversing Mirror

I am a solo caravanner with a 4WD towcar (without rear mounted spare wheel).

When reversing to hook up, it would help me if I could look in my driving mirror and see an image of my towball and how close it is getting to my caravan towbar.

At the moment I push a cane into the ground close by the drawbar and because I can see this through the back window I can "judge" how close I am but it obviously risks a damaging bump or a hernia from manhandling the caravan onto the towball.

I wonder if a downward facing removable suction mirror would work, but I don't know what sort or where to get it.

I can't afford CCTV and monitor just for this purpose! 

I would like to know what others do apart from begging help from others.

Thank you.