Just some ideas here as I've not decided on materials for mine yet but what helps is to think of the construction like this (see pic).
The original back looks to be made in 2 halves (or mirroring the look of the lower cushion and a little like the wall pads) with a noticable seem line seperating the top inch from the back inch (I'm guessing this back part is approx 2 inches deep).
This opens up a few ideas as to how cozy and durable the back cushion could be. If you plan to use the seat and sit in it, a completely solid back may eventually be a pain in the... back. But one idea is to have the detailed top inch solid (fibreglass or soft wood, etc) and the back inch in a high density white or light grey foam or neoprene - stick them both together and then weather. This means there will be cushioning and some movement when fidgeting about, answering your comlink or checking the hyperdrive status. I'm not sure how comfy this will be lol, as my chair is my office chair.
The idea can also be used for a fully solid back. If you want to make a buck in wood or clay and go as detailed as possible, then if it gets damaged over time you only have to cast another top inch and re-attach to the back part.
This could also be reversed, by having the top part in 1 inch white neoprene or some kind of closed-cell high density foam/rubber and the back inch in a solid material, as most chunky foams only dip about an inch. But this way around means carving out the rounded corners into a 'half bullnose' shape/profile and that could be tricky, unless there's a tool for rounding corners that will work on softer materials.
Some soft back material searches I've been looking at include: High Density Seat Foam, Closed-Cell Neoprene Sheet and Closed-Cell Polyethylene Sheet. Some 'closed cell' foams already have that irregular bumpy look which is good for weathering to sink in and cling on to and is easy to add all the dents and scrapes to with a hot-wire or soldering iron.
I'm still looking at different quick and easy ideas for the base materials though.