If you plan to jam it up in the reeds or grass mats and shoot out of it as a blind, many of the existing plans will work. If you want to be able to jump shoot ducks as you paddle through the creeks, you're going to need something wider. The problem is, to get something that feels really stable to someone not used to paddle craft, it's going to be very slow, unless it is 17-18' long, in which case weight and maneuverability become problematic.
Jacques' comment about a flat bottom not always being more stable can be true, as flat bottom boats typically won't draft as much water as rounded or v-bottom boats. This can make your center of gravity higher as less of the boat is submerged.
From my experience, bottom width is the greatest factor for stability in hand-built boats, as we aren't doing to more complex hull shapes like mini pontoons, etc. For a boat stable enough to shoot out of, you will want at least a 30" bottom, 31"-32" would be better. For a boat to be fast and efficient, with a lot of "glide" you want a length:width ratio of 7:1.
My pirogue is 14'3" long with a 25" wide bottom and it is much faster than production boats I've been in. It's actually too fast for thick swamps, as a half hearted single stroke will glide you 50 yards right into a tree, there are no brakes.
I can stand in it but it is a balancing act and requires concentration, I have yet to shoot from it without sitting in brush. I built another that was 16' long with a 27" bottom and it was more stable but not massively so, and the extra weight was noticeable when accelerating, glide factor was nearly the same.
If you won't be dealing with wind and current, and aren't in a hurry to get where you're going, a Cheap Canoe widened to 32" will do the job. Maneuverability is another trade-off determined by rocker and the amount of weight you will carry. For good tracking, you want the bow and stern just barely submerged. For good turning you want the bow and stern slightly out of the water to allow the boat to spin around it's middle. My boat has 2" of rocker and it's perfect for rivers but not quite enough in the swamps, 3" would be better. The increased maneuverability will be worth it for you and as your boat will paddle slowly you will be fine using smaller, lighter strokes which will help it track straighter than really digging in and pushing hard for speed.