Hi Chas
Yes thin plasticard is a good way to go if you can handle a little extra weight up so high, the scallops should be easy enough to cut. In addition, to simulate the framework under the fabric, carefully bend the plastic over a straight edge then straighten it out again, doing all outlines of the transverse members and then the fore-and-aft central bearer. I am unfamiliar with the structure used on the launch you are building but the same principles may apply. If weight is an issue you could use umbrella fabric, holes for lacing are easily made with a soldering iron, and a hem can be carefully made just by folding the fabric over a light smear of super glue. The plastic will look far more realistic however.