Yea, make sure there is enough room to get your skimmer in and out of there incase you need to remove it for some reason, and make sure you can get the collection cup off.
Maybe someone can back this up, but I think having the carbon in like that where the full flow of your sump/return pump is going over it doesn't work too well. Usually carbon or phosban want longer exposure, slower flow. May think about picking up a little media reactor and running the carbon in that, and that will give you some extra space to work with in the sump. I don't think the carbon will be able to do its job effectively in that setup. But, maybe a pro will also chime in on this.