I would rather not buy a $100 ND400 filter that I may or may not use multiple times. To get a taste for the options, I want to see if it is possible to create one inexpensively. I am not looking for the highest quality(i.e. Lee or Singh-Ray), but acceptable would be nice. What material would get me somewhere in the neighborhood of ND400 or so, and be easy enough to mount to a 77mm filter thread?
This post on DIYPhotography.net illustrates using a #12 welding glass as a ND filter. Indeed the filter is green in color, but the author is able to correct with whitebalance. Note that welding glass is available in 'strengths' from 1- 14. 14 is dark enough that folks use it to watch solar eclipses.