I think the HDPE would need reinforcing to be strong enough. For weight you could cut a rectangle section out of the board near the lower trailing edge and fill with lead. It could be captured on all 4 edges with pins and would be flush with the board. Lead is 750lbs cu ft so it doesn't take much. Probably a few pounds would do on a small board. Just for reference I had 40lbs of lead holding down a 32' sharpie centerboard...the board was made of 2"x10" wood planks drift bolted together and approx 10' long x 3' wide.