Wood pallets can easily be reused as a building material: they are often either free or very inexpensive to acquire, they are very strong, and they are easily transported. You can construct a watertight animal shelter in an afternoon out of a few discarded wooden pallets or skids. If you are accustomed to completing home building projects yourself, you will probably be able to finish the entire shelter using only construction scraps that you already have lying around.
Attach two of the wooden pallets together along two adjacent edges using L-brackets and screws. Adjust the angle of the two pallets so the resulting A-frame will rest on the third pallet.
Attach your A-frame structure to the third pallet, which will serve as the base for the shelter. Use more L-brackets and screws to complete this stage. You may need a friend's assistance to lift the A-frame onto the base.
Cut a triangular piece of luan or plywood to fit the opening at the back of the structure. Attach the back directly to the pallet A-frame with screws.
Cut a smaller triangle of luan or plywood to brace the front of the structure---allow plenty of clearance for the animal to comfortably enter the shelter. Attach the front triangle to the pallet A-frame with screws.
Staple a tarp over the roof and sides of the shelter to waterproof it. Stretch the tarp tightly over the frame as you staple and fold it over the corners to prevent any loose edges.
Line the interior floor of the shelter with a carpet scrap or mat.
Always exercise caution when operating power tools.