Marine Goop
you want
Marine Goop

I've been using this stuff for years.
its great for a million little things

for your pannier repair, I'd probably use a pice of innertube, and just glue into position with Marine Goop
it will probably last longer than the rest of the pannier

I glued a Cars-r-coffins patch onto a messenger and I pulled it off with a pair of channel locks after 6 years

you can also thin any of the "Goop" products with Toluene
you just have to do it in a metal can, or glass jar
toluene will melt plastic
use in a well ventilated area.

when I apply Goop, typically I use a piece of wire, like coat hanger, old spoke, etc...

work with it, you'll the hang of it