I used a chemical called 'Mod Bond' and it chemically 'melts' the Lexan on a molecular level to chemically allow the molecules to flow and interlock or something like that. They use it for bonding Lexan made Fishtanks, so it definitely is capable of producing a firm bond, but you must file off the UV/scratch resistant coating of the Lexan for it to bond to it. Yes, the bond chemical will not soften the UV coating. It is very thin, so it does not work well with gaps, unless you grind up some Lexan into shavings or powder to act as a filler, then the very thin Mod Bond chemical softens the Lexan and and allows you to pack in the shavings and powder (will solulate into the chemical) and fills in gaps. Also, if you study this pic:
you can see how I also cross-drilled the lense and pinned it in using little pieces of thick wire to hold the lense while I was glueing it. I believe it also gives the bond extra strength. Then after bonding everything, I smeared a thin layer of CLEAR silicone around the inside of the fixture around all the seams, and that is like a second insurance that if for some reason I developed a crack in the Lexan bond then it still wont leak into the fixture. And so far, a year or so later, and I don't have any leaks! I got the Mod Bond from modernplastics.com which is where I also got the MR-10 Lexan (.250 thick).
Another couple pics:
Hope this helps