I've posted this before, but I'm reposting after using 2" spot mirrors for a number of months and finding how effective they are in providing awareness of things around me, especially when I am working or transporting where cars approach from the rear.
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The mirror in the image is pretty old, but it attaches in seconds with double stick foam tape to each of the bucket supports plus it can be "tilted" to put the focus where you need to watch.
Amazon has
similar adjustable mirrors at 4 for $6