Inspired by the commercially available and all-too-pricey balance ball desk chairs out there, I decided to make my own with the ball I already I own, some PVC, wheels and a few tools. The ball can also be very easily removed (simply lifted out), so you can use it for working out. This is an inexpensive and easy project that will (hopefully) force an end to slouching and work those abs while 'working' or in my case 'studying' (yeah, not so much studying as fantasizing about bookshelves and LED throwies while on Instructables).
Note: If you have back/neck problems or at all concerned about how this might affect your back/neck/whatever, please consult your doctor or physical therapist. I am not responsible if injury or further injury occurs to you or anyone else as a result of this DIY guide.
Step 1: Supplies
Balance ball (I used a small sized one, but I think a medium might be better; the large might be too high for the desk)
4 12" pieces of 1 1/4" PVC pipe
4 1 1/4" PVC elbows
PVC primer & cement (I have ones with the brushes inside; if you don't have that, add brushes to your supply list)
4 2" twin wheel casters with stems ... the ones that have the metal piece that the stems snap into (brakes are a nice feature too)
Spray paint (or duct tape, which is what I had on hand and used ... but probably does not look as good)
Saw (if you need to cut the PVC down)
Sandpaper (I had 150 grit, which worked fine)
Drill & 3/8" bit (to fit the wheel stem, so adjust bit size as needed)
Step 2: Building the Base (Part I)
Next, put your pipes together to form a square, making sure it is even (i.e. place it on a hard floor and check that all corners are making contact). Take the sharpie and at each pipe-elbow connection, draw a short line across the two pieces and number both so that you can line things up correctly when you make the permanent connection. Disassemble your PVC square.
Locate to a well-ventilated area and put down some paper towel. Give your PVC primer and cement a good shake and brush the purple primer on the ends of the 12" pipe and the inside of the elbows, covering where they will be overlapping. Place these on the paper towel to stay clean and dry, which should be pretty fast.
For this part, you want to be fairly quick so that your cement doesn't dry before you get your connections made. Brush the cement on one end of a 12" pipe and inside the corresponding (matching numbers) elbow. Connect the two pieces, pushing the elbow on as far as it will go and matching your drawn lines. Hold this for about 10 to 15 seconds. Do this for each connection. The last one might be a little tricky to close the square. This should also dry pretty fast.
Step 3: Building the Base (Part II)
If you have a preference for which side is the top and which the bottom for the base, place the bottom facing up. Drill a hole in each of the four corners (in approximately the same position at each corner). Take the metal pieces that the wheel stems snap into and stick one in each of your drilled holes, and if necessary, hammer them the rest of the way down. You don't need them flush--just tight and each in as far as the others. Snap your four wheels in.
If you are using duct tape like me, start wrapping. I did a pretty quick job of wrapping the duct tape, so I'm sure you could make it look a lot nicer if you felt so inclined.