I would split it into 3rd's ...using the picture on the far Right as the example.....
Looking at it from a side view, the front 3rd would be the mans head to where the neck begins, the front half of his legs including his curled down fingers and that portion of the ground directly below the front half of his legs. Envision a vertical line drawn like I described. The palm of his hands should be in the middle 3rd with his fingers in the front 3rd due to the under cut.
The BACK 3rd is fairly simple...just imagine a line dividing the front half of the bench from the back half with the man sitting on it.....the line would go over his lower back and hips.
Brian is absolutely right, that's a three piece mold in the making. Don't forget the registration keys (another name for the locking pieces). And don't forget to apply a release when you make the different parts.