Seat bags are nice, used them for years when commuting to carry work clothes/shoes/lunches. No chance of scratching tank either. Great for day trips plus a little bit of back support/rest is always welcome.
Regularly shop at a full blown military surplus store near me and picked up some nylon and canvas bags that are trulY multi-purpose. Water proofed by coating with beeswax and linseed oil - works great and is truly low cost.
Nelso Rigg makes decent stuff, like the looks of the saddleman tunnel bag.