Need advice on building cheap pontoons

My pontoon style houseboat has about a 5 foot overhang in the rear. The weight causes the stern to hang low. The previous owner strapped (literally with nylon tiedown straps) on old plastic dock floats. I want to redo it better by fabricating some auxillary pontoons.

A couple of notes, the boat is on an inland lake. The 50 hp motor pushes the boat at maybe 10 mph tops so it doesn't plane. These are auxillaries meant to last 5 to 10 years. If they start to leak the boat won't sink. They don't have to look pretty. I'm just looking for an alternative to the rusty 55 gal. drums everyone else uses for the same purpose.

My idea was to build the pontoons out of 3/4 plywood held together with PL Premium Polyurethane glue and stainless screws and then paint them.
Questions:
Should I use pressure treated plywood or will CDX work just fine? Using 3/4 plywood I can't justify marine plywood. If I use CDX do I need to coat the seams with epoxy resin and fiberglass tape or can I just paint it? The interior too? Is it necessary to cloth the entire surface?

I tried to attach a drawing of what I have in mind.

 

Gut feel - sounds fair enough with exterior grade plywood.
I am not sure of what CDX is, I presume "particleboard", which I think would be lucky to last 2 years in constant immersion.
If you cost the option of epoxy and cloth on the sides and bottom, versus just paint, you might find it might increase the cost by 10%, but increase longevity by 100%, especially if you allow for the occassional minor impact over the years. Dont forget you will use a less paint with epoxy.
The big unknown is the quality of the plywood you get hold of. I would certainly at leat paint the inside.

 

I have some pretty nice structual marine plywood 3/4" x 4' x 8' for $69.50 if that helps. For the cost difference...which shouldn't be much. You'll get a much longer life out of your project.
Let me know if i can be of any help.
Regards,