I was in the local brick and mortar Woodcraft store this past Saturday and looked through their stacks of 1/4"/6mm baltic plywood and wasn't able to find a single flat sheet from the piles and stacks they had on hand. They were probably good enough for bulkheads and centering rings but nowhere near acceptable for making fins.
I ended up buying a 48" length of Incra T-track that I plan to use for a modification of my drill press auxiliary table. Going to cut a notch out of the rear right side of the table to allow the hand crank to clear the table to raise or lower it without having to remove the auxiliary table. The T-track will inlet into the table near the center to allow the fence full front to back movement. If I were designing an aux table from scratch, I'd install four t-tracks for clamping and such.
25sqft for $40.99 and includes shipping. This is where I ordered mine from to make the fins for the Jericho.
Very well packed and quick delivery. I ordered the 12"x30" 10 pack, every piece was flat, no warping.
+1 on woodworkerssource.com - I am very pleased with the 1/8" birch I ordered from them, have already cut 8 fin laminates (faces for G-10 cores) from them. Good quality product, fair price, fast service.