I made a version a couple of years back, using a wooden pole and a door mirror extension (towing mirror). The pole sits in the towbar-mounted bike carrier bracket and the mirror is attached to it such that when I sit in the driver seat, the rear view mirror and mirror-on-a-stick combination combine to show me the towbar. It isn't perfect but it was free as I already had the bike carrier, and the other bits were hanging around in the garage.
Before that I'd experimented with a wireless video baby alarm with the camera magnetically attached to the tailgate (and removed once hitching is complete), but the screen on the cheap one I used was a bit rubbish and in any kind of sunlight was too difficult to see.
If the budget ever allows I'll definitely get a proper reversing camera fitted...