If you are having success with the wet dry then you are doing good. Is that the only reason you want a sump? I do agree that its an excellent piece of equipment to use, no doubt about it.
I'm not sure how much space you will have with those dimensions though.
I'm working on a new 60g build and I only have 23 inches to work with so I was gonna go with a 15g tank and maybe make my own sump. That size should be fine for a 60g not sure if it will be good with a 90g though. I also wont be putting a fuge in it so I have more skimmer room.
Are you planning on making this sump yourself? using a regular tank or building a tank?