There are four hot / cold plates. They are ~6" diameter. Three are like the one shown here and the forth is the cylinder plate
which includes support for the cylinder. All four plates are made of 1/8" thick 6061 aluminum. Holes to clear eight #4 bolts are drilled accurately at the edge. The plates are 1/8" under a full half circle to allow for a 1/4" insulator
used to attach the hot and cold sides.