Sit in a cockpit for more than a few hours and you too will appreciate a cushion under your butt.  I prefer support for my back, but that's me.  I'm OK with sitting on 3/4" high density foam if I sit there for any long length of time because it leaves a higher back rest.  But before long you will find me wandering around the boat to get out of the hot sun or to promote circulation to warm up when its cold.  I have no problem standing on the companionway ladder leaning against the hull, especially if that gets me in the shade.  I can stand there for hours in a rolling seaway. 

These box cushions are not quite square, as you can see from the dimensions, but they shouldn't be too difficult to make.  You will have to allow for the slope of the gunwales when considering the thickness of the cushions.  Unless you find it easier to pick up crackers, etc through the gaps behind the cushions.