There's several ways to secure your gear to a SUP (or kayak). Recently I've been using cargo nets on the deck which is a great way to secure one or several items at once. The netting is also great great for rough water paddles or surf where gear can shift or be knocked off hte board.
I found 12" square nets at a hardware store nearby in the auto department for about $5. I've had to customize the size of the net to fit the board and my sea kayak correctly. One tip is to string bungy or thin rope around the perimeter of your netting for a more secure fit. You can also use the netting as an anchor for tying gear ontop of the net.
Attach the netting to your board via leash plugs or stick-on tie downs such as NSI or EZ Plugs.