When I started out, I put a reflector on the rear bumper on both sides, just far enough out to be able to see them in the rear view mirrors, which turns out to only be a couple inches. I found that I wasn't as 'long' as I thought I was. I probably don't need them anymore, but I broke one off and I replaced it anyway. It's a quick and easy reference to glance at.
What I am using is those reflectors that are on the end of about a 3 ft. fiberglass rod that people often use to mark their rural driveway entrance. Lumber yards have them, and they don't cost much. You'll find them around where the signs, mailboxes, etc are located. I just cut it down to about 6-8" shaft and mounted them with plastic electrical cable supports, whatever they are called, the ones that are molded to fit TV or network cabling.