I added a tarp made from Milliput to the cargo bay. It could show more wrinkles, but apart from that I am happy with the result. To get such a large flat sheet of Milliput, I used two sheets of baking paper, a lot of baby powder and a rolling pin, which worked great.

The pictures below show the tarp almost finished, the straps of the side covers, the attachment to the holding loops and some seams are still missing.

I also mounted the cargo bay to the frame without any problem, and added some splashes from thinned oil paint.