Keep it simple. Rebuilding a 20 into a no frills fishing/work boat.

[Basketcase]

So its getting to be time to order a fuel tank for this boat. Looks like I have a few options. Poly, aluminum, or stainless. Which one would be best suited for an under deck application? I do not plan to have an access hatch. Im planning on installing it and putting the deck on top with an access for the fill/sender etc. I know if I ever need to replace it that I have to cut the deck up but I'm not afraid of that. Any thoughts?