is it this one?:
i would ensure you have adequate cooling airflow around that fixture. at 280 watts (in that small of a canopy) it is going to be pumping out heat. leaving the back open should be good. if you are concerned about light leaking out the back, then you can cover the bottom half of the back and leave the top half open. heat rises, so the more you can get out at the top, the better.
if you enclose the entire canopy then a fan will probably become a necessity.
if at all possible, leave yourself the ability to add a fan at a later date, if you find out that your cooling isnt adequate.