It's not a commentary on your engineering ability or your construction skills, but this thread reminded me of
this site. Actually, I think you thought it out a little and everthing looks pretty square and parallel.
On another note, off topic, if your canoe is made of Royalex, it may be worth more than you think. A tree fell on our dock this winter and I had to get replacement values for one of our canoes. I noticed the prices for Royalex boats were going up, so I checked around. Royalex will no longer be manufactured as of next month. I paid $700 for my boat about 7 years ago. Insurance gave me almost 3 times that.
Plastics News article.