I´m looking into the same issues building my own little tanren device. Did the towel and rocks in any way alter the response of the tire ?
I used a tire for a Toyota four runner (My truck) so it is about waist height. I see a lot of tanren using motorcycle tires. I am not sure if it changes anything. The rocks on the bottom of the tire helped keep it stable and helped to stop the ringing. The towel folded and bunged to the top of the tire. This helped to quiet the impact and reduce the bounce on the bokken. It doesn't look as nice as the tanren stands but Its just in my back yard so no big deal. It works great and there is less bounce and is a little quieter now.