Note that while specific materials are mentioned here, substitutes are readily applicable. For example, using Coat hanger wire instead of galvanized wire as well as using duct tape in instead of epoxy/SuperGlue. You don't even need to use the rectangular mirror. You can use the circular type or make a rectangular mirror from a circular one. in all cases, a convex mirror is most appropriate. Flat mirrors just don't have the coverage of a rectangular convex mirror.
Step 1: What You Need
1. Special Mirror (see image and description below). Typically found at auto parts stores like AutoZone
2. About a foot of 14 gauge galvanized wire (or similar substitute like copper wire). I find aluminum and copper wire is not stiff enough. It seems to always get bent out of shape. Galvanized wire holds its' position very well. Wire from a coat hanger would probably work just as well!
3. An alligator clip (or other suitable substitute). Alligator clips with teeth work the best but they will mar a surface. Other "toothless" clips seem to always slip out of alignment.
4. Epoxy (or other suitable adhesive). Five minute epoxy seems to work best as it provides a bit of time to assure correct placement as well as provide a large adhesion area. Superglue tends to pop loose when twisted.
5. About a 3 inch by 2inch bit of strong self adhesive tape.
You'll note that other than the specialty mirror, everything else can be found around the house. The mirror itself can be cut out of a circular mirror. Why rectangular? Well it allows for a wider, single glance horizontal view of what's behind you while eliminating a lot of vertical view that is not needed in a circular mirror.
Step 2: Mirror Disassembly and Preparation
After removing the mirror, I found the reflective coating exposed. I covered this with clear packing tape. The tape will provide a very good surface to glue the wire.
Step 3: Attaching the Wire to the Mirror
I used 5 minute epoxy which has held very well. This might be bit of overkill but it sets and cures much faster than say an RTV solution. Super glue needs a lot of contact between the surface and the wire. It also breaks loose when side loads are applied to the wire or mirror.
Anyway, how you attach the mirror is not nearly as important as attaching it in the first place.
Step 4: Trial Fit
To do this, put on your helmet and hold the mirror in a position that allows for unobstructed viewing to the front yet allows you to quickly shift focus to the scene behind you. My mirror is positioned 5 inches directly in front of my left eyebrow. The view places the juncture of my left shoulder and the left side of my face just at the lower right corner of the mirror. This provides maximal coverage to the rear. Turning my head very, very slightly in either direction significantly expands the area viewed.
Hold the wire on the helmet at the point where you will attach the mounting clip. At this stage you can start bending the wire for initial positioning.
Once you have found the best position, allow for about an extra inch and cut off the excess wire. This extra wire will allow for mounting to the alligator clip and provides a bit of slack for fine tuning.
Step 5: Attaching the Mounting Clip
If you used copper then soldering is a good alternative. Don't forget a heat sink to prevent excess heat from melting the mirror!
Step 6: Completed Assembly
Step 7: Mounted Up and Ready to Ride
Total project time: 30 minutes.