Take a cue from the regular awning manufacturers: add a bracket on either side of the window and also at either end of the awning rod. Attach a short aluminum rod to hold the awning in the extended position and add a tie-down strap in the middle with just a little tension on it to hold it in place.
It's like a standard window awning, but no big roller tube or springs. It would look great and be functional as well.
Solar Tech Energy Systems, Inc.
Victron Solar Components and Inverters, Zamp Solar Panels, LiFeBlue and Battle Born Lithium Batteries, Lifeline AGM Batteries