Our pack does the regatta indoors with just the Tigers. The trick to avoiding a mess with plastic gutters, is to put a bucket under each end and let er drip. At the end of the race, use a mallet to knock the end cap off, and be ready to catch the water in an empty bucket.
To attach the keel and rudder easily, mark their position on bottom of the boat with a pencil, then use the pointy corner of the metal keel to make deep punctures along the lines. Do this on a table to avoid puncturing your hand. The parts will fit snugly into the new slots, and glue is often unnecessary.
For painting the boats, each scout gets a square of scrap wood with a drilled hole in the center. The boat's mast can be put into these block (upside down) while paint is applied and dries. We put a light coat of clear acrylic on each boat to prevent coloring the water.
To keep the sail out of the water, cut a short piece of plastic straw and place it on the mast before installing the sail. We let the scouts cut the sail with scissors, and find that reducing weight from the top makes the boat more stable.