The best solution is to use an allen-head bolt and a nut, and use a drill with a diameter slightly larger than the size of the allen key required to remove the flats from the bolt, and use a grinder to file the flats on the nut down to a smooth cone shape. Make sure you have your seat height exactly where you want it before you damage the fasteners because you will have to drill out the remainder of the bolt to remove it for adjustment.