I have had to soak for 3 weeks to a month. I weight them down with glass jars filled with water in a rubbermaid tub. It'll work, you just have to have a lot of patience.
I would not use a galvanized screw, galvy is just a zinc coating. Infact I would not use any kind of metal screw.You could also try attaching a piece of plexy glass (acrylic) to the bottom of your wood and put sand on top of the plex glass.
Make sure to use galvanized steel screw so it won't rust.