Lucky for you, the British made pretty detailed stowage diagrams for their instruction manuals, and although these aren't M4A3E8s (or M4A2E8s, since Fury is actually an A2), it should give you a good idea of what kind of stuff tankers carried on their vehicles in WWII.

https://i.imgur.com/xvfv55S.png

https://i.imgur.com/tYQdxSC.png

https://i.imgur.com/TNfrGLf.jpg

From these you can see that there are indeed sleeping bags carried onboard, but they are stored in stowage bins. It would be pretty unfortunate if you carried your sleeping bags outside of a tank with no protection at all, since odds are that by the time you get to your camp they will be very dirty and wet.

The object is most likely a waterproof tarp to cover the tank itself, since it would not be vulnerable to dust or rain.

However, these are only the official stowage guides. In practice, tankers carried more thank they were authorized. This is especially noticeable in North Africa, where supply lines could be long, but it was seen in Europe as well. The longer the campaign went on, the more unauthorized brackets, racks, etc. were installed on the tanks for the crews' personal convenience. A lot of the time you see extra crates full of ammunition or belongings attached to tanks, as well as spare wheels or track links, sacks of this or that.

As for trying to simulate this material, you are better off asking in r/modelmakers. Fabric is going to look way too rough at scale, usually putty or a tissue soaked in diluted glue is used.

Sources:

D. Oliver M. Starmer The New Breed Pt.1

http://tankarchives.blogspot.com/2017/08/necessary-improvisations.html