Boat Mods must keep it light

I have just purchased a Alumacraft 16' jon boat (70" beam and no center seat) that I want to extend the front deck and add a floor. The issue is I only have a 9.9 hp outboard and want to still be able to get on plane with 4 people (10-12 mph). So weight is critical, I have made many things from wood, but I fear it is too heavy. If I use alluminum, how do I know what kind or how thick to buy to support my weight between the ribs on the floor (or to walk on the deck). My secondary question is how do you attach those seat base plates (I see on bass boats) to the thin flimsy aluminum bench seat without being able to get under it to add some type of support (please remember weight is critical). Any other materials, or out of the box ideas would be appreciated. Thanks for your help!

 

Any help would be appreciated... I thought about using a bunch of coolers secured togather to make a platform (in the front of the boat as a deck) strong enough to walk on and it would have lots of storage also. I know this is a little crazy but it would acomplish my goal of being light strong (and boyant as a bonus). Thanks in advance for your help.

 

Could you post a pic marked up to show the proposed deck area and the approximate dimensions? This will encourage more meaningful responses.

I see several downsides to the cooler idea (although it certainly is unique):
Coolers aren't that light.
They aren't cheap.
They probably won't fit the space that well and will need to be secured.
In the event of a swamping, you wouldn't want all that flotation up front; especially with a shallow draft jon boat. It would likely sink by the stern.
They won't provide a continuous deck surface.

Once we see some pics we can suggest some suitable solutions. My initial thought would be to build a torsion box (similiar to a hollow core door) out of 1/4" Exterior Luan (interior egg crating) with 3/8" Ext. Luan skins top and bottom and suitable solid or plywood edging. Depending on the necessary span and height above the floor you will likely need some sort of center support.

 

Pic of my boat

Here is a picture of what I have, I am trying to extend the 4' front deck another 4' to make it a larger platform to walk around on and mount a couple more seats for fishing and casting. Thanks for your help.