Another technique (which works well for me) is to use duct tape every six inches or so to pull the two keel strips together, and use the plywood U clamps with spring clamps to keep everything flush to the forms.

Like Jay, I also nail one one the keel strips to the forms. I use 4 d finish nails. I am anal about making sure the keel stays where I want it. I also use book-mirror strips on the keel strips. Any slight waves in each strip are then cancelled out as they are pulled together. Helps keep things straight.

Hope this helps. Have fun! Mike