By far the best adhesive I have found is Essex U-418 quick cure, primerless auto glass urethane adhesive. It is black in color, has no smell, and is very strong. I have used it in 5 homemade scopes. If you use it be carful not to get any on your hands, as it is impossible to wash off. It is used in glass and auto body shops, and I have even "glued" in metal patches over rust holes on underbodies of cars I have refinished with excellent results.
Ask for it at auto glass shops. I have used it on many types of material and am impressed with it's strength and bondability to various surfaces. When cured it is a little harder than silicone. Wonderful stuff. Ron P.