Some rear racks are designed to fit normal road bikes, with a front arm that attaches via the rear brake bolt, and adapters to fit onto the quick release axle. I would guess you wouldn't want "too" much weight on it, since the 3-point mount isn't as stable as a touring bike's 4-point mount.

For example: Journey Uni-Fit Mk2





I haven't tried any of those, but I do have a seat post mounted rear rack. It doesn't hold very much at all, not enough for a 4 day trip. I even extended it farther back 5 inches with a piece of 3 inch wide 1/4 inch thick oak from the hardware store.

It's this topeak rack, without the side frames. The back edge of my saddle reaches as far as the front tubular stop, so I can't go too high, and it's only 15 lbs weight limit.
I've loaded a jacket, extra gloves, lunch, and a pair of walking shoes on top, and that filled it up. And the seat post clamp sticks out just far enough to brush against my thighs, which was annoying at first.