Hi Les I built the Guillemot at 17' and found it to be very maneuverable. I paddle in the Mississippi river and other rivers in this area. In fact I some times wish for a stronger tracking boat when I paddle in the big river. Now that I have a year or more with my Guillemot I just love this boat.If you are set on making it shorter I would think 8" to 12" would be as short as you may want to shrink it.

You may be concerned with weight and car top loading if so it can be built light. I would recommend a light weight build of 4 oz glass on the deck and inside the hull and deck. 6 oz on the hull bottom. No need for a second layer if you launch from sand or in the water. I would use a wood block in the ends instead of an end pour. Use a stapless build and treat the epoxy like it costs a mint. I would use 3/16 by 5/8 strips try not to add a lot of stuff to your boat. If you don't plan on over night you may not want a front hatch just a float bag stuffed up in the front.

A new point of view may be a skin on frame. That would be light like load with one hand light. The cost if that is a concern is much less with a skin boat.
I could go on for ever about building boats its so addictive.

I almost forgot I used an external strong back for all my builds it sets up fast and can be very accurate. It makes altering the design easy if you want to.

Have fun with it Clayton P.