Need to improve wet canvas holder

Last week my new Guzzi V7 Stone got the first maintenance finished as well as the European side stand was installed, which was the last piece I was waiting. So I took it to a long hwy ride first time passing Bellingham and got to Lummi Indian Reservation. I painted on Fishermans Cove near Whatcom Chief Ferry terminal to get to Lummi Island.
This was my first plein air session this year (since previous Guzzi Nevada didn't run) but it went ok. But when I came back home I found the surface of the canvas had hit the top part of the canvas holder multiple times, losing some paints. You can see it clearly on the bottom right corner of the photo below; losing the beach color. I need to come up with something to keep the canvas staying at the bottom by next time I go out painting.