Clean off the 2 side tabs and bottom tabs to allow it to come out. Drill and tap what's left and use screws and washers. Small amount of hot glue or tape to hold in place. I have never done it but just a thought.
The edges of any of those little dots or swedges I think they call them, that overlap that seam need to be carefully cut or ground away with a Dremel tool and then that little back piece will come out. And then the lense drops out. I'd just use some JB Weld in place of the swedges when you reinstall everything.