I had thought of floating panel exactly as you are discussing but in the end I fit bulkheads tight to deck and to slightly below shear and left a 3/16-1/4 gap at lower section. I've glassed the upper part in and will use silicon caulk along bottom seam to seal it up. My theory was the bulkhead will definitely not move, the deck gets good support and the areas of likely impact on the bottom will have the ability to flex a little. Have no idea if really will make any difference. Would like to hear how your floating panel works.