I have seen a few people making a custom shelf for the rear. I took some of the ideas and work I have seen and created my own version. I basically wanted something to help hold up the soft top from drooping and settling into the rear trunk space when the top is down.
I also wanted something that matched, is light and is easy to install and remove. I figured good old wood with a matching carpet was the easiest and least expensive.
I started with a Home Depot run for a 6' piece in a 12" width board. Placed it on top of the tub and cut out a basic pattern:
From the pic above you get the idea. The shelf will sit on top of the tub and be snug inside of the soft top sail panels. The holes line up to the holes in the tub. Cut outs are for the rear seat belt plastic things (real technical!)
I also grabbed a thin roll of black carpet from Pep Boys. Lined it up, cut around the board and fastened with small staples.
Placed it into the rear and fits like a glove:
I have 4 bolts that I need to add to the shelf to secure it. They go from the top of the shelf through the tub and nutted underneath. It was late last night so I just plopped it in and left it.
All in all it cost about $25 in wood, carpet and bolts. Pretty simple but effective.