Newmar , at least in my unit's vintage , uses a metal channel 2" high with 1/2 " flange top and bottom , two sections hinged in the middle and at the mount plates on each end .
I'll see if I can post a picture.
Once on the Newmar site looking for other parts I saw them for sale.
But I'd tend to build one myself rather than order, parts would be less than the cost of shipping.