PVC clamps work great. Easy to make and very low cost.
http://gallery.bateau2.com/displayimage ... 8558&pos=2
http://gallery.bateau2.com/displayimage ... =717&pos=0
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I know this is no help to you but I'm building the fs17 and the plans call for 1.5" square cleats on stringers and frames and the drawing shows them on both sides of each. The stringers are 3/4" so the total width will be 3.75". Now that seems like a lot of lumber and glue, but it should be strong. ...
He has a separate album with pictures of the seat build.