Use card board from a legal pad. Cut off the end that held the paper. Cut 2 one inch strips from the long end of what remains. Cut a small portion off each end at about 35* angle. Place the two pieces evenly on top of one another. Place a piece of tape at the ends, securing one piece to the other. Bow the two pieces away from one another. Insert a 1/4"x1" piece of left over card board between the two pieces to hold the sides apart.
Step 2: Stitch bottom
Place vessel on top of remaining cardboard. Trace outline of bottom vessel onto cardboard. Carefully cut out traced portion.
Step 3: Stich bottom.
Secure cut out portion onto the inside bottom of the vessel using tape.
Step 4: Glue
Mix about a table spoon of school glue with some baby powder. Put in enough baby powder to make the mix as about as thick as peanut butter. Smooth mixed thickened glue into all inside seams.
Step 5: Paint
Paint the whole thing. Probably an oil based paint would be best for water proofing.