When building the base, pick a shape and size appropriate to the size of the sculpture. I'm going to build a base with two sections. The top section is approximately 1/2 to 1 inch larger than the sculpture on all sides, and is cut from a poplar 1"x6" board. The bottom section is approximately 1/2 inch larger than the top section on all sides, and is cut from a 2"x8" oak board (if the base will be painted or stained very dark, the different woods won't matter).
Once you have the sections rough cut in the shape you want them, mark on the top section where the sculpture will go and where the supports to attach it to the base will be. Mark on the bottom section where the top section will be placed. Then, on the bottom of the bottom section, using the top section, mark approximately where the supports will be placed.
Using Carpenter's Wood Glue, glue the sections together. Clamp the sections in place, using another board on the top to keep from damaging the base.
While the sections are secured with the clamp, drill screws through the bottom of the base through to hold the top section (the picture above is slightly skewed, the screw should go through the larger board to about 1/4 inch from the top of the smaller board). Make sure that you do not put the screw where the braces for the sculpture will be. If the wood you are using is prone to splitting, drill a hole slightly thinner than the screws your using first, being sure not to break through the top of the base. On a base this large, use at least three to four screws. Make sure to sink the screws further into the wood, and not flush, so they will not scratch furniture when the sculpture is finished. Remove the clamp, wipe off any excess glue, and let the base dry for 24 hours.