Hello, mate! You'd be our first AUSSIE on ORR.net, I'm pretty sure!
I think here we call them tonneau (pronounced TONN-ee) or just bed covers. I guess a coverlet would be the same thing. I just wanted to keep my things dry and out of sight without having to remove the 5th wheel hitch. And be able to tow without leaving the cover at home.
Perhaps you need to seek a US based supplier? What are you using for a tow vehicle?