most people do just grow in a closet man... i know i always have... if your worried about tidyness or pests or whatever, just get a roll of some heavy duty plastic sheeting. hardware stores have it. they should have black, clear and white. you will want to get black, to keep the light in/out, and you will need white, to help with reflectivity.
i cut them down to size and tape them back to back, then just line the part of the closet your using with it. dont forget the floor! and make sure its light tight, i sat in mine with the lights off in it to see.
now if you get a bigger light, just make sure that its suitable size for the space you have. is it a sliding closet or a swinging door one? either way, its not the best idea to use the plastic idea with a big light, but as long as its far enough away from the light itll be ok.