Originally Posted by hoganc
...the sunbrella material will not just wrap the foam insulation in the curved shape without puckering, wrinkling, or folding...
The trick to no puckering, etc, is to cut a CURVED pattern. Should be easy enough to make your own.
Get a piece of paper (I recommend freezer paper - available at any grocery store) and hold up under the rail (might need another pair of hands for this). Trace the shape of the rail, inside and outside curve. Then calculate the necessary width of the pattern (rail diameter + overlap for velcro + any seaming). From the CENTER LINE of the shape traced, add on the necessary width. TADA...you have a pattern.
If you use freezer paper, you can then cut out the paper pattern and iron it onto the sunbrella fabric (the little bit of wax on the paper will melt and temporarily afix the pattern to the fabric, but will easily pull off when you are done cutting). Pattern will be sturdy enough to reuse several times.