I currently use the Home Depot method from the above answer and it works fine.
Follow the measurements correctly though. Home Depot was out of the smaller pieces to make the handle so I bought 1 inch longer (5 inch instead of 4). It sometimes rubs against my legs now. So smaller handles are better in this case.
Tim's describes how to make a cheap kettlebell alternative for $10 using standard weight plates and a few things from Home Depot's plumbing department:
* One 3/4" diameter X 12" pipe threaded on both ends
* Two 3/4" diameter X 4" pipes threaded on both ends
* One 3/4" diameter pipe "T" fitting
* One 3/4" floor flange
* Recommended: One spring clamp
For assembly Instructions, click here and watch the full video.
I am living in Germany and found it hard, to find the parts to build my own T-Bar (as Tim described). But the concept of a "universal kettlebell" made me look for something similar.
I found this: Barbarian Kettlebell-Handle (at http://www.tradoria.de/hantelbaenke-und-zubehoer/barbarian-kettlebell-handle-144994601.html).
It has a real kettlebell grip and you can add standard weight plates (though a little more expensive, about 30 €)