I ordered 8' lengths of 1"x1" aluminum extrusion and 2"x2" L-bracket to cut down into parts for the aluminum frame.  The frame was designed to be a 3' cube, so there wasn't much to be done to the aluminum extrusion besides cutting it to length.  The L-bracket required a little more work as it had to be cut down into 1" thick brackets, then have two holes drilled in each side for screws.  The process wasn't difficult, but it took a while to make all of the needed brackets with a few extra just in case.

    The reason I chose aluminum extrusion is because it is readily available, relatively cheap, and easy to use.  Assembly is simple once you have the brackets, requiring just 1/4-20 T-nuts and screws to put it all together.  I also set up the linear rods for the x-axis motion along the top of the frame.