When I daydream about something like this, it is so that I can keep my hips straight (the way they are when people are standing or lying flat).

In my daydream, I am lying flat "on" ("against"?) a board, as if lying on a bed, but the board is at an incline of about 45 degrees. The board is of a slippery material like formica to make it easy for me to get on and off it, which makes it important for there to be a "ledge" at right angles to the board for my feet to "stand on"("rest against"?) so I don't just slide down the incline.