Once the extension board is in place, figure out the exact length you want the pipes to be, place a bar clamp across the board, so the bar would stop the pipe exactly as long as it should be. now you can simply load the pipe in, two maybe three at a time, hold it tight against the bar clamp and cut. They all will be exactly the same length.
Interesting note, to make the blocks I stacked up three pieces of 5/8" MDFB. I tried to screw them together for about a half hour, and then thought...Duh, I can just duct tape them together. Worked perfectly. Duct tape, is there anything you can't do?