I have been following other BD-4 builders and their seat mounting schemes...the plans call for a short length of tubing with 2 tabs welded to the back and attached to the side of the C-channel. A slot is cut in the front halfway down the length of the tube. Another smaller piece of tubing is welded to the end of a pipe that runs the width of the cockpit. The seat rails sit on these two pieces of tubing and the seats sit on top of these. I have used 2 boat seats attached to 0.125 steel plate that have adjustable rails attached. I will get 7" of movement out of these seats. Cutting the individual pieces, deburring, welding, placing and ensuring the assembly is square and removable has taken considerable time. Here are a few pictures.