Window A/C Unit - Install

Need help with little window ac unit to be installed in horizontal slide window. I want the window to still be openable, (new screen(s) or modification?) with insulated plexiglass (or another window)? to keep cool above where window ac will be. I will use tension rod to lock closed. Better to make metal frame? use support bracket shelf? I dunno. I've been using portable ac units but want the floor space back and don't need excessive $$ units for such small rooms.