No bedpost, no problem. You just need to improvise.
You’ll just need a hairbrush with a smooth handle, or better still, a dildo or vibrator.
Then find a flat hard surface that’s about thigh height, one that you can comfortably straddle. A bedside table or a chair might suffice. Then rest your penetrating object on the surface beneath you, and cup your hands around it so it’s vertical and just below you.
The idea here is that the object that penetrates should stay still, resting on the hard surface, and you do the moving as you ride up and down upon it.
I think many have found the bedpost story so arousing because it involves a very different way of masturbating. Usually when you play by penetration, you remain still and it’s the object that moves in and out. The bedpost scenario is exactly the opposite, the intrusion is immobile, and playing involves working your legs, moving your whole body rather than just your forearm.
Bonus idea for the naughty: hold the protrusion in one hand, and spank your bare bottom with the other as you ride…