This is probably the single most breakable item on an RV. Just one time out when the flap is not secured and it is alongside an interstate!
Some years back a SunnyBrook owner started making them for sale and I bought one. It is a very simple design as was described above and it has lasted probably 10 years and still going strong.
Just thinking out loud? would it work to make a piece of sheet metal and hinge it at the top with some clever use of velcro and seal at the bottom with either velcro or clips like are used on screen doors to hold in the glass window?
One other thought; could one be made with a 3D printer? Although I suppose the average RVer doesn't have one in their rig!