Here's a pretty compact idea.
http://www.baysidemarineservices.com/livewells.asp
Ton of them out there but many are way over priced... Hey, it's a boat. Break Out Another Thousand, right? Not sure what type of leaning post you have but I guessing you might fashion something similar with a lot less work than cutting up 4sq. feet of your new deck...I've seen some short & squat tanks that might fit under you leaning post depending on post model... Good luck.