Marine Grade Plywood?
Just to throw out some ideas for consideration. Given that you are lake and river sailing you might take a look at the work of Antonio Dias. Tony is an old friend and a really wonderful designer. For some reason his website is not up at the moment but he has designed some wonderful small and traditional cruisers that offer a nice mix of seaworthiness and performance. I have sailed on a number of his designs and they have proven to be very good boats.
Another thought would be to look at Dudley Dix''s design for the marconi rigged Cape Henry 21. http://www.dixdesign.com/ch21.htm
This is a very clever 21 footer designed for plywood construction.
If you were interested in a more modern interpretation of a small cruiser, this one has a daggerboard with a bulb for greater seaworthiness, you might look at Dudley Dix''s Didi 26 design. http://www.dixdesign.com/26didi.htm