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Why mirrors when I can simply turn around to look back? That's true for standard upright bikes where mirrors are more luxury than necessity. But on a recumbent they're essential: your field of view is similar to that in a car, so you need similar means to cover the blind spots behind you. You just need to find a place to install the mirror.
Mirrors that fit handlebar ends are readily available at bike and motorbike shops, so finding them is not a problem. The tricky part is to find a spot where the mirror works and doesn't obstruct anything, and a practicable compromise between viewing angle and range. If you find it, you get a reliable "back eye" that can't be lost or forgotten at home. I wasn't that lucky with my unusual USS setup, so I have no experience to share.
If there is no good spot on the bike, the mirror can be fixed to a glove or to your hand. I tried a palm-sized round panoramatic mirror purchased at a car shop (lorries have these on their big flat mirrors to see wider angles), with a loop of clothing rubber band sewn on it. The resulting gadget is enough for a quick check before a left turn (the hand is going up to indicate anyway), not for a permanent surveillance. There is one big drawback though: the mirror is too convex and shrinks things too much. It means a comfortably wide field of view and no trouble aligning it to position, but it also means that approaching car headlights look like two tiny sparks, and you can only see them when the car is very close to you. It's OK in a town, but if the cars move at 90+ km/h, it's already too late to react when you finally see them. So a conclusion: hand-mounted mirror is not a complete dead end, but a too-wide-angled one is.
It sounds weird and looks even weirder, but it works surprisingly well. Hang the mirror on some "antenna" before your left eye, slightly above the horizon so that it doesn't obstruct forward view. By being so close to the eye, it provides a wide viewing angle even though it's flat and quite small. Shaking is not a concern, a head is the most stable bit of your body and if it bounces so hard that the mirror shakes, you worry about your consciousness more than about your vision. My amateurish prototype reflects the whole width of the road a kilometre behind me and all it needs is a slight turn of my head. Helmet-mounted mirrors can sometimes be found in bike shops. Glasses-mounted ones are rarer, but making it yourself from an old spoke and a mirror shard is quicker and easier than placing an order in an e-shop :-).
All three illustration photos on the right were taken at the same place and time and the camera was where my eyes are (not so easy with the goggle-mounted mirror, I had to remove the glass), so you can compare what can be really seen through these mirrors. That red car is about ten metres far.