For your reference, Mark.. I just bought a piece to make a new companionway slider. The plate is 30 inches wide and 33 inches long. It is 1/2" thick gray plexi, color #2064.
I bought my piece from E Street Plastics for about $120. My local plastics guys did not have anything that thick in stock. E Street custom cut it to the 33X30 size. There is a small wood cleat on the front end that will not allow the plate to slide aft until the sea hood is removed.. On the aft end, there is a notch on each side to allow the slider to come aft until it covers the companionway boards. The notch is 1 1/8" wide and 5/8" deep (fore/aft) that notch forward edge is beveled at about a 45 degree angle so that it kinda matches the inside of the slider cover "horns" at the aft end.
Hope that helps