DIY Sailboat for about $350

I am planning to build a simple two man sailing dinghy this summer. I built a boat before, a knockoff of a Sunfish, but I regret making her too heavy. Floated fine, handled sluggishly but then very forgiving; ultimitely too hard to put in alone or throw on the car-roof to take to the shore.
My design was fine: center-of-force vs. center-of-resistance wise. And I rigged it was too small a sail, a 6.5m windsurfer sail, using the wishbone rigging and all.

I want to sail but I need a lighter boat, don't want to spend $5000 on a Laser, and want to be able to launch singlehandedly so it has to be under 200 pounds. I'll bet my first boat weighs 600 soaked.

There are plans on the web for simple boats made of a single sheet of 4'x8' plywood. Will lumber ever go metric in Canada? Anyway, these are called one-sheet-skiffs. Not quite the cardboard fun boat used hilariously to race during carnival, but just this side of disposable. I'd like to get 5 seasons out of the hull, and will rig with Tyvek sail. Apparently, logo free Tyvek is not available from Home Hardware or Home Depot, so I'll have the stupid logo but plan to currupt it visually with paint and morph it into a seacreature or something.

My mast will be a tree of some kind. I'm just going to go into the woods and find the tree that looks like my mast and take it. I will bend it and make sure it is supple. It needs to be about 14' high and about 4" at the base.

The hardest part is making the plywood cutting plan. I'm using a sheet of 3/8" marine plywood for the hull and another 3/4" for ribs and transom.

My secret hope is my design will be so simple that each spring my small sailing club will build a few boats and then race and sail together in the part of Malpeque Bay by Lennox Island & Green Park.