There are lots but you don't want one. I tried four seatpost mounted racks. Three rubbed on my thighs when pedaling, and the other wasn't strong enough to keep a laptop containing pannier out of my spokes.

Use an adapter replacing your skewer or a pair of P-clamps for the bottom mount. You can use padded P-clamps with hose clamps above and below to maintain their location. Use P-clamps or a seat clamp adding eyelets for the top.