The Topeak rack/bag is a little more complicated. you buy the rack and the bag separately. The seatpost rack clamps on to the seatpost and can be used on every type of bike available. (there are road specific versions that are lighter but any will fit any) No braces, eyelets or other stuff needed.
The bags come in different sizes and can have fold out (unzip and fold out) mini paniers (a small frame must be used to keep them from swinging into the tires) or an unzip and expand upwards top.
The bags have side pouches for tools, bars etc and the main compartment is large enough for a sixpack.
Not an everyday bag but good for special occasions. Want it off the bike, quick release lever like a seatpost clamp does that.
Topeak reps contact me here for my shill reward