I've now finally sorted the cargo out for the goat by using nothing more than some off cuts of foam board and duro epoxy putty. I firstly glued various shapes of foam board together to represent a load then rolled out a large sheet of putty. This was then draped over the cargo and sculpted to fit the cargo bed.
A suitable pack and a cooler box were added too and pressed into the curing putty. The whole thing once dry can be removed for ease of painting later. I will also be adding a couple of other bits of stowage to the cargo bed later.