I converted a little canvas side bag to a proper saddle bag, its a little baby carradice.
Before
Original bag by steveo_mcg, on Flickr
After
Saddle bag from the rear by steveo_mcg, on Flickr
Its only just big enough for my mid week commute, shirt unmentionables, travel towel, sandwiches and morning can of coke. My trousers won't fit so they'll need to come in on a Monday in the Carradice. Should be able to convert any small bag like this. It should do for a bar bag on long weekends.
Bar mount by steveo_mcg, on Flickr