How thick is the Lexan? Matt could probably give a better answer, but I think Lexan is a cheaper quality plastic. I don't think it will do very well with the heat of metal halides. My bet is that it will melt/start burning which in turn will release toxic fumes into the area. Also, having the top of the tank covered with halides will create a greenhouse effect causing the water temp to greatly increase. That and you will have to constantly clean the covers from salt creep.