Can't tell the size of your ship, but if it's a foot long, a footy keel would probably work. There are posts and plans here and elsewhere on the web. That size keel would be about the width of your jib's foot, and a foot deep. I generally make my keels wider, say 2/3 the width of your main boom. But my boats sail slow, and don't generate the lift that a fast moving Footy racer generates. The wider the keel, the better you'll work to windward, but the surface area will slow you down in a light breeze. It's perfectly acceptable to have more than one keel, for different wind conditions.
For testing ballast, I use a small pill bottle taped to the bottom of the keel, filling it with lead shot or pieces of automobile wheel weights. At a minimum, there should be enough righting force to bring the boat back up if she gets knocked down (sails flat on the water). If your sails are cloth, there will be a surface tension suction, so you will need more weight than you'd see just testing the boat on land.
The deeper the keel, the more righting force the lead will have. So, if you find, by experimen,t that you need more lead than you want (due to buoyancy problems), deepening the keel will let you reduce ballast.
Adding styrofoam blocks to ensure she won't sink if knocked down is a good idea, especially while you are testing. Tape them to the floorboards for testing (use waterproof tape, of course...electric tape is not waterproof, at least here in the US). Boats have sunk on their maiden voyage, so be cautious.