For what you have in mind pretty much any bag will do and it doesn't have to be custom made and fit perfectly because let's face it, there's always going to be crap that you'll have to place outside the bag. Your laptop, camera equipment, shoes, etc. For that very reason I actually prefer them to be a bit smaller than the actual panniers.
We've been using plastic bags to prevent t3h smudge(tm) but have now purchased two large and one small Hein Gericke inner bags for a few bucks each. Much more comfortable than plastic bags and obviously not fitted to our Tesch panniers but still excellent.
"Life is not a journey to the grave with the intention of arriving safely in one pretty and well preserved piece, but to skid across the line broadside, thoroughly used up, worn out, leaking oil, shouting GERONIMO!"