does anyone suggest any changes?

Ok so I just built this 2x4x4 FT box for my setup... I have 4 T5 bulbs and 2 150w equivalent cfls. A baby fan atm ... I'm thinking ahead and trying to figure out how and where to install the carbon filter and fan... the whole box is made of hardboard and 2x2 pieces of wood. Also what do you guys suggest I add? Still room for growth in here, I plan of cropping two plants in this space.

depends on your goals. as it is the only thing i would do different is get a lightmover if budget allows it. This will move your t5 set up back and forth creating equal bud growths over all the tops. otherwise the buds to the sides not directly under the light will suffer. Once you get some experience expand to somewhere else much bigger, or this will limit you to an average of just an oz or 2 every yield