Keep the Center brace intact and make 2 lids, this way you can acsess one side at a time and not subject the Viv to a great outflux of humidity and air temp, this will also help keep any critters that may stick to the sides from makein a jump before you can bring your hand 4 feet to the other side. Createing two lids will also alow you to place misting and or watering heads in such a way that you wont have to move or ajust them everytime you want to go in and fix or remove something. Aside from that you will be having at 8" max depth 40 gallons of water bouncing in at a nice 340 pounds heh, better safe then sorry imho
