over head sump tank could it work

i have converted my tropical tank into a marine tank all is going good with it but i do have a external cannister and have read they arnt that good for reefs, if i had planned it better i do admit i would have put a sump tank under the main tank would have been a bit of a job but it could have been dun but my first question is is it possible to have a over head sump tank it would be 3ft long and 8 inches wide plus could i put a lid on it to what are you thoughts on this idea