While it has been some time since I last applied a pair of covers, I used Weldwood contact cement. (Bob Drake used to include contact cement with his covers, but stopped doing so because of its flammability.)
Be sure to clean the underside of the rubber to remove any release agent from the molding process and rough it up with very course sand paper. Glue small sections at a time starting with the outer edge to achieve a straight horizontal line.
When finished with the contact cement, use a drop of black liquid rubber to fill the holes on the undersides and to fill in the horizontal seam between the rubber and steel under the outer edge. For originality's sake, the steel foundations should be pained flat black.