Now that I had my photos and materials, I built the heart shape using very thick wood for the sides. I felt that the wood should be hollowed out to make the painting lighter, since a solid block of wood is soooo heavy.
The guys at the local "Wood-Mart" helped me to find a strong blade that would cut thick wood without breaking.
I cut the face of the heart out of primed masonite. I like to paint on this stuff because it is very smooth.
I used a waterproof wood glue to fasten the face to the sides.
A pile of books and a clamp put enough pressure on the box to give the glue a good seal.
After the glue has dried, I will refine the shape of the boxes. I can cut the best heart shape when both wood pieces are solidly fastened together.