I plan on changing my stationwagon to a pickup soon and will be doing similar. I was going to use steel tubing that I’ll, cut, bend and weld to fit. You should see where the attachment holes are already in the pickup bed sides? I was going to get it galvanized afterwards.
As for the cloth top I’d look at your local sailmakers/boat cover makers. I have a friend who runs a business in Southampton called Sail Style that I was thinking about asking or I was going to pop up to Exmoor Trim (famous for making rag tops for landrovers) to see if they could make a custom one or adapt a landy one to suit. They have quite an array. Theres also a charity that employs disabled people near us called PLUSS, they do tons of stuff but have a fabric manufacturing place where they make up boat covers, camper van interiors etc – they’ve done tonneau covers for one of my sports cars before, maybe you’ve somewhere like this near you?
Let us know how you get on and stick up some pictures!