I have made them numerous ways. The most expensive way was from billet aluminum. If you need drawings I have my blueprints still somewhere.
The cheapest way has been to simply weld them up from steel.
So for the lower bushings, I've used a piece of steel tubing with the same ID as the mounting bolt, capped at either end by a steel plate...essentially these plates are in the shape of a thick washer. The top mounts were welded to the crossmember top, with a nut welded to the underside of the new plate to allow bolting to the framerails. The neat part about this method is the fact that for racing I offset the rear lower holes, and shortened the top mounts an equal amount (flush with the crossmember). This lowered the car an additional 7/16"-1/2" without losing suspension travel.
For my ITB suspension I'm not allowed to weld to the crossmember or suspension parts, so I made the mounts from steel but made them bolt-in.
Here you can see the top mounts on a crossmember for a rally Ascona I'm building. They're just tacked in place at this point.