Although my campler longflap saddlebag is great for commuting, it's sometimes to cavernous for short day rides.
Needing something smaller, but not wanting to shell out much money I found my old Topeak rack bag which has a plastic base which slides onto a special topeak rack. Not having the special topeak rack anymore I wanted to see if I could mod the bag to fit to a normal rack!
Well, it was actually quite an easy mod. inside the pack there is a foam liner on the bottom, under this you'll see 4 screws. Undo these and the special topeak track base is removed leaving 4 holes suitable for longs strips of velcro or bungee cord. My solution was long length of bungee cord with a dog lead clip on either end threaded through the holes in the base of the trunk pack, which I then use to secure to the hooks at the bottom of my rack.
I've not got any pics yet. OK, so it's not as good as a carradice rack pack, but it'll do meantime!