Unless you are trying to keep everything original... just make a new cover... if that is an option for you.
Some of that diffused plastic sheet that you can get at HD would work fine. Cut 3 pieces. 1 for the top, and 2 for the sides, and using a very small, 1/2" x 1/2" x 1/16" aluminum angle to connect the pieces, rivet the 3 pieces together to cover the fixture. Paint the angles to match whatever. You might need to just attach a small angle bracket inside the fixture on either end to secure the new lense to.
Just a thought!
