Brian, I ran into the same problem of setting up the 9 degree angle. So I cut the scarf at 30 degrees using my table saw. When I epoxied the joint, I did both sheer clamps back to back so at least one side on each would be totally straight. Haven't run into any problems strengthwise. I can move the sheer clamps by lifting one end, and watching them flex evenly throughout. When the glue cured, I planed the top and side that will be glued to the panel totally flat. They worked well as a batten.

Since the sheer clamps aren't load bearing members, not having a full ratio scarf probably won't impair the boat's strength. I'd just cut them with the miter box and make sure to epoxy the crap out of them. That, and when you're clamping them, make sure that you're pushing the joints together along the length of the strip as well as perpendicular.

Free tip: When you measure twice and cut once, wait before you cut and measure again. I trimmed my sheer clamps to length today and they came up about 6 inches short. Oops.

Bill