There are several DIY canoe dolly/trolley/cart plans via a Google search.

Are you sure you want to go this route though as there are many canoe carts for almost any price range. For example, we have a cart similar to this one here that we picked up second hand for $40.00. I am sure if you looked on Ebay, there would probably be some that will fill your needs. Also you can search on the Paddling.net classifieds section.
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Earth and sky, woods and fields, lakes and rivers, the mountain and the sea, are excellent schoolmasters, and teach some of us more than we can ever learn from books.

John Lubbock