yea its plenty for 1 plant, try hanging the light fixture then making it so you can just raise it as the plant gets larger. your gonna have 0 growth sideways. and even adding anything to the sides (mylar, lights...) might be pointless, once the plant is in there, the canopy will cut off all light to everything underneath it.
another option would be to top the plant, and have 2 main colas, probably your best bet.