Have a Softail Slim S w/ motherwell fender rack. Looking for suggestions for bag to strap onto rack,large enough for weekend trip.

 

Believe it's called Ortlib and is sold by Aerostich. Been using it for about 10 years on adventures from Alaska to all over the West Coast and still does it's job and is waterproof. I've got the same fender rack as you on my Fatboy. I do use some duct or electrical tape on the fender where the lines I use to tie it on rub.