If I were to do it I would use insulation foam for the coffins, and connect them together and attach it to a back brace with pvc pipe. There are lots of examples of back braces/harnesses so you'd have to look around and find a type that works for your brother. I'm not too sure how thick you would want the coffins, but using foam core or insulation foam you could use spray foam to create a type of sandwich to lessen the weight but still allow for a thicker coffin. And also, I would use pvc for a few reasons; easy to curve, it's light weight, and with pre–made connectors it makes traveling to cons and assembling cosplays easy.
Ryuk | Wings | 30%
Soul King Brook | All | 20%