The skeg protector is more for your prop. If you hit something hard enough to break off the skeg, it won't matter what else is on it. Skeg protectors are well worth the money to protect your prop at $60-90 for a rebuild and $90-140 for a new one, and that's aluminum not stainless.
My 19' Alumaweld with windshield only has flotation under the floor and maybe four cubic feet in the back. Rated for a 120 I think. You could probably get away with cleaning up/adding insulation under the floor and maybe just attached to the top or bottom of those side spaces.