$.02

make the hip braces a vertical plate of ply about 5" wide, that's about the width of your average adjustable hip braces. Make three vertical slots like Eric Schade does for looping the back band webbing. Glass both sides of hip plate if made from 4mm ply. Attach WELL. The hip plates are not structural but need to be attached well for the leverage a back band applies.
All the suggestions showing minicell for back rests are fine but assume a strong set of postural muscles. If you are using a backband it needs to attach to something and attachment to sheer clamps as CLC recommends really is not the best arrangement, put water into stressed hole of soft Cypress and you'll see it show up eventually on the outside. Sit on back band and it's pulling on 1/2" #8 screw,,,not a smart set-up IMHO.
All minicell does work but the large volume in the cockpit is,,in the width/height of the cockpit so you're really not displacing that much given all the space forward.
Here's why I like hip plates,,I like back bands, the space behind the backband is still useful, the space outside the hip plates is useful, adjustable hip pads allow for a wide range of body types and any minicell hip brace that will actually hold you well will get torn from sitting in/getting out. They aren't like thigh braces that are engaged only when in the kayak. The form fitting seat/hip brace combos are real nice for one size paddler and a narrow coaming. To pad out that 17" wide coaming so that you don't flop side to side can be addressed with a nice form fitting seat but the actual hip/hip spacer will get jammed when you get in/out.
With a nice thick minicell seat jammed hard between the hip plates with slots on the side it can't move fore/aft. With adjustable nylon covered hip pads (Salamander) on the hip plates jamming down on top the seat can't float out. With the back band attached via slots in the hip plate you don't need hardware. The setup can all be removed in five minutes to a bare cockpit but not require carving anymore minicell except what's in the seat.