A quick low cost solution for a horizontally wide angle view of what's coming up behind you. The total cost of parts not found around the home is $3.99 (for the mirror). Compare to an off the shelf rear view mirror starting at $14.99. Also, this mirror provides a wider horizontal view than the circular off the shelf mirrors!

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

You will need the following:
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

The first thing to do is remove the mirror from the plastic housing. You don't necessarily have to do this but I find it removes mass (bulk) which reduces the amount of weight hanging out there on the end of the wire.

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

The back of the mirror will be concave so the straight part of the wire will be raised above the surface a bit. I slightly bent the wire so it would conform to this surface. This will reduce the amount of adhesive (and hence weight) than if the wire is raised above the surface.

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

Once the glue has set you can now perform a trial fit to find out how long the wire needs to be.

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

I leave this step to the reader. An image of how I attached mine is provided (I crimped and glued).

If you used copper then soldering is a good alternative. Don't forget a heat sink to prevent excess heat from melting the mirror!