First the Ursack is advertised as bear resistant not bear proof, nothing is bear proof
FAQ, have there been any failures?
We have sold bags in 50 states, Canada and assorted foreign countries. So far, there have been no reports of a bear compromising a standard size IGBC approved S29 AllWhite. Two Majors have been compromised–a bear tore a one inch hole in an Ursack Major–it is not clear if it got much, if any food. A few (about 2 per year) earlier (pre 2014) S29 AllWhites sewn without Spectra thread had the seams ripped. The S29 (green, no ripstop, heavier fabric) introduced June 1, 2004 was the model tested by SIBBG. The fabric has been nearly perfect, but user error (not cinching and knotting Ursack tight) resulted in a few bear incidents. That’s why we changed the design in the AllWhite: to make the cinching and knotting easier.
There is no such thing as a bear proof container. Rangers have reported to us that even the hard sided canisters are broken by bears on occasion. A recently published book, Speaking of Bears, reported that one Yosemite bear rolled about 25 canisters over a cliff to break them on the rocks below. source
Given the available information. You can make a bear resistant pannier, as you would any other pannier, use Spectra fabric and Spectra thread. Sewing would also require an industrial grade machine, capable working with the Spectra fabric and thread. I would suggest contacting the Ursak Company to custom make your pannier as probably the most cost effective solution.