It might be lower cost to get a metal tank made than a custom plastic one.

You might contact Dura Poly ( http://www.durapoly.com/fab.htm ) to see what a tank to your measurement would cost. There must be other places that could also do it. A note, I have never been in contact with Dura Poly. Just found them on the web.

I would also look at a couple of smaller tanks instead of one bigger tank. But then you are always faced with being able to carry the max amount vs redundant systems.

Small boat placement of stuff is a bear! I KNOW!!...
Good luck!

Greg