Easy to make a good detachable one, wish I still had a sewing machine! Cut a piece of canvas to the shape of your bonnet, maybe even another as the top, stitch together with a piece of fleece or whatever between. Fit a diagonal piece of webbing across each of the rear corners, about 250mm out from the corner. Do the same for the front, but add a slide buckle in to the middle of it. Bind all 4 edges with 20 or 25mm binding. Lift bonnet, place mat on top and slip webbing loops across the bottom/rear corners. Do the same at the top/front and cinch up the buckles. The buckles and loops then are hidden under the closed bonnet. Job done.
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