I have seen some that mount to the wheel axle.

Many people use the seatpost mounted racks for 'bikepacking', but they often design their touring setup to be ultralight. If you want to carry heavier stuff then the seatpost racks are not ideal. Sape goes for P-clamps - these can work for securing the rack to the seatstays to prevent fore-aft movement, but if you use them to connect the legs of the rack near the frame's dropouts, they will eventually break.

What sort of bike do you have? There may be other options but it's impossible to know without details on the bike.