Keith, your assumption is correct. You can also use a partially inflated football, but you can't tie it to the board very well.
For the bouy, get a bungee cord about three feet long. Cut off one end and remove both hooks. Now slide the bouy on the bungee. The remaining metal crimp on the uncut end should keep the bouy from falling off. Now tie the cut end to the middle of your strap and you're good to go.
Attached, are some old pics to show what I mean