Finally removed my bow mount, and all the old crusty plywood. There are still two bench seat in great shape. They will stay. How can I structure the boat to support a casting deck from the second bench forward with a hatch for the battery underneath?

Not taking anything away from this forum but you really should check out tinboats.net. it is a site full of aluminum boat builds and will give you a lot of good ideas. 

12 minutes ago, riverbasser said:

Not taking anything away from this forum but you really should check out tinboats.net. it is a site full of aluminum boat builds and will give you a lot of good ideas. 

already did! wanted to check here too!

I used wood, but it's heavy. if I had time, and money I would use aluminum and rivets.

Aluminum angle and channel. Not to be a smart ass by any means. Check out what they have laying around at the scrap yard for materials. They will be glad to help you. Cheaper than buying new materials. Aluminum stays and struts are the way to go. Like your project. 

Aluminum & weld it

I framed mine out with 2x2s.  Plenty strong enough no longer then they need to be and light weight.    Your front piece of plywood will sit in the v shape and doesn't need much support it can't go down in the v. Does that make since.  I used floor underlayment for my boxes and just glued them together with adhesive caulking. They only hold my tackle boxes. As for the battery that will take some frame work. Mine is still in the back of the boat. 

I'm on tin boat forum. But I really didn't take enough pics of the build.