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The thickness of the block will determine where your butt goes which determines the trim of the boat. Usually being about 4 or 5 inches off the back of the cockpit will float you trimmed out about right. This is a good place to start and you can adjust later to compensate for excessive plow or stern heaviness.
There are a couple of elements to consider in a back block. One is that this piece will usually require the largest pieces of foam in your arsenal. The higher up you make this piece the more it will give you the support in the lower back that you need, but the higher you make it, the harder it will become to get in (and out) of the boat. So build it high and shave it so that you can get in and out easily.
If you have a monster chunk of foam you can build the back with one piece. If not, you can do it with several. The pictures above are of a block that is made of 4 pieces.
Start with the bottom. Don't worry about the contour until later and don't worry about the top piece until later. Cut a 3 inch wide area out of the middle of the bottom piece so that it can be slipped in the boat around the wall. Make it as deep as the size of your foam will allow. This will help keep it from coming out while driving down the road. Once this is in place glue the top piece on and then shave a nice contour to the whole thing.
If you are short on foam you can skip the tabs that stick back and just glue pieces up against the stern wall. This will of course eliminate easy access to the stern of your boat. One good reason to have the removable block is to be able to access the stern for a place to put your sandals.