One important factor is the material that you use for your enclosures; concrete has some distinct advantages structurally, acoustically, and aesthetically. It is very stiff and heavy--great for supporting the speaker drivers without changing their vibrational dynamics. It also damps out vibrations well, so it doesn't contribute unwanted rings and unpredictable tones to your sound reproduction. Equally important for something that sits in plain view all the time, it has a unique and interesting visual appeal.
So dust up on your concrete mixing skills and lets get to work!
Step 1: Find the Molds
For the inner molds, a kids' rubber play ball works great. This one is especially good because the nubs prevent it from touching the outer mold in a large area. You can adjust the internal volume by adjusting how far the inner mold sits down into the outer mold. Put sand in the ball so that they don't float in the concrete.
Also, don't forget that the molds are about to get very heavy, and will be difficult to move when they're full. Think ahead about where you set up your work. The surface should be as level as possible because the top of the wet concrete will level itself, and you want it to match to the height of the mold all around.