What you are calling hip plates actually derives from the struts that fiberglass boats use to hang fiberglass seats. They automatically worked out as a brace to keep your hips from sliding side to side.

Yes, you can pad out the sides with foam to achieve the same. I usually make "hip struts" and run them from the deck to the floor of the boat. I use the space between the plate and the hull to store my pump. You should have some sort of hip support - but not too tight. A hands width of space is good. You want to be able to twist and turn a bit in your seat but not be sliding from side to side out of position. if you decide to learn to roll and even for leaning the boat with various techniques, they are a necessity.