As far as the string-pod, I found it helps to tie a little dowel or other item to the end of the string. It helps with gripping it to the floor. Also, if you make about 6" short of eye level, it will give more stability. I've tried this where I couldn't use a monopod or tripod in the Cologne Cathedral, and gotten some nice shots, being able to handhold at 1/4 second.

I wish I could be more help, though, as your question is one I know I've wondered about.