Depending on the size of your toy sailboat you can cut a thin strip of craft wood, or make a strip from a stir stick, and fit it to your mainsail as a boom. In smaller scales you can use bristles or some other stiff, round material to make a boom in a suitable size.
To make the boom, use a steel ruler and a craft knife to cut a thin strip of craft wood the length of the bottom edge of your mainsail or slightly longer. Sand the stripwood carefully to round the edges, or you can sand them square if that is easier. Use a folded piece of sandpaper to sand one end of the strip so it has a slight cup shape that will fit against your round edged mast.
Apply a thin bead of glue to the forward edge of your mainsail (the edge at right angles to the base of the sail) and press the sail up against the mast. Leave to dry. When the sail is glued securely to the mast, run a bead of glue along the bottom edge of the sail, and set a small amount of glue on the end of the boom where it was shaped to fit the mast. Glue the boom to the mast and the bottom of the sail. Allow to dry. When the boom is securely glued to the sail and the mast you can trim the boom to length if necessary.