Run the center of the wide side of the 4-by-6 through the router to cut a semi-circular groove along the length of the board. The piece of piping with the same diameter as the radiant heating tubes should fully fit within the groove and just be flush with the top of the board.
Lay the heavy piece of piping on a hard surface --- such as a concrete floor --- then lay the strip of sheet metal over the piping. The piping should run the fully length of the sheet metal and be exactly in the center.
Set the 4-by-6 with the groove aligned to the pipe below the sheet metal. Start at one end and put pressure on the top of the board causing the sheet metal to bend around the pipe. Continue applying pressure down the entire length of pipe to bend the whole length of the metal strip.
Pick up the board and pull the pipe out of the groove. Gently pull the sheet metal out of the groove and it should maintain the outline of the board's surface. The radiant heating pipes will be fitted into the sheet metal groove, then the strip is mounted to the bottom of the floor boards.
Repeat the bending of the metal strips for each plate you need.