The material was one eighth inch 316 stainless. Backing plates of the same material were used bedded on top of 1" mahogany blocks worked to match the contour of the backing plates. I don’t recall the length of the plates, but I wanted them as long as possible. I believe I stopped them just short of the shelf that runs the length of the boat. The tops of the chainplates on the exterior protruded above the deck just long enough so the turnbuckle could be attached without any interference. The hole at the top of the chainplates will be determined by your individual rigging, but there is a formula for determining the placement of the hole to allow enough remaining material around the hole for structural integrity. (It is: the diameter of the hole plus half the diameter of the shroud of remaining material must be left – taken from "Understanding Rigs and Rigging" by Richard Henderson.) If I recall correctly the forward most chainplate was forward of the main bulkhead and the other two were just aft of the bulkhead. The middle one was directly in line with the center of the mast. If you can find a Pacific Seacraft Flicka near you, that is what I used to determine the placement and material to be used. I placed mine the same distance apart as the Flicka. You must have the top of the chainplates bent at an angle (sorry I don’t remember the angles anymore) to have the rigging run fair up to the tang on the mast so there will not be any unnecessary bending or twisting of the rigging. Also install toggles at the top of the chainplate, between the chainplate and the turnbuckle,  to allow for any misalignment. To find the correct angles I sighted from the top of the chainplates to the tang on the mast using a protractor. I’m sure this is not the most accurate method, but it worked well for me. Of course the chainplate is not there yet for you to take the angle measurement, so you have to decide where they will be and how far above the deck they will protrude to get the point at which to sight from the deck up to the tang. I taped cardboard mockups of the chainplates in place to take the angle measurements. To get the chainplates flush with the side of the boat I cut off sections of the coaming, installed the chainplates, and then expoxied the cutoff pieces back in place after installation. I had a local machine shop in San Diego do the machining of the chainplates with directions and a drawing from me. I’m sorry I don’t know where that drawing is anymore. We thought the modification was very strong and an improvement to the design of the boat. The boat went very well to weather and pointed as high as Catalina 25’s with which we sailed. Sadly, we don’t have the boat anymore. Lance