We have made our own in the past. If you have machining and welding capability, bore out some steel rod to slide fit over the shaft. Then mill a flat(s) through the bore to match the depth of the flat(s) on the shaft. Then weld flat stock across the flat(s) to engage the shaft. Alternately, you can visit your local dealer parts counterm, see what the van door motor drives, and you may be able to adapt that for your needs. Your choice.
