This is an after-the-fact instructable, so I do not have very good step-by step instructions..
1) magnifying lens .. I found this at a local surplus store, but these type of lenses are fairly common.
2) 1.5" diameter ABS plastic pipe coupler .. I used some scrap that I had lying around, but you can find ABS pipe couplers at your local plumbing supply store.
3) two mini key-rings .. optional.
4) elastic chord .. try you local sewing supply store.
5) sunglasses strap .. you can find these most anywhere.
1) Using ABS glue or epoxy, glue the magnifying lens into one end of the ABS coupler
2) On each side of the ABS pipe coupler, cut two 1/4" holes and slip the 2 keyrings through the holes
3) Cut two 3" lengths of elastic chord and attach the sunglasses strap
4) Smooth the edges of the end of the coupler that will be against your face.
Sorry for the lack of details in this instructable, but I hope that I have given you enough info that you can create your own version.