Read reviews for the Interior Mounted Cockpit Bags by North Water as submitted by your fellow paddlers. All of the reviews are created and written by paddlers like you, so be sure to submit your own review and be part of the community!
I mounted one bag behind my seat (not enough room at the thighs). It has worked great there, it clips in/out quickly and securely and the double zipper works well enough I can zip open/close with either arm/side. Very well made (all Northwater products I've seen are). Can't comment on it if mounted at the thigh.
Minor nit: I think it would work better with some kind of hard shell to keep it's shape.