Because this is a larger window than the stock D shaped window, there is some concern about drumming and flexing. I made some .025" angles 3/8" x 1" to strengthen and brace the perimeter of the window. These will be final drilled for #6 machine screws when the polycarbonate sheet is installed. Need warmer weather for that. Drilled out the holes to #30 for installation of blind rivets.