I assume you're really looking for something similar to the wedge bolt and scalloped bushings on this stem that clamps down on the handlebar center sleeve in this case??:

If there's just a hole going through past your seat post.....
If that's the case, I'm nt sure if those are easy to find as they are not that common. As seemed to have been noted already, you might have to fabricate something similar from aluminum and steel bar stock and a bolt if you cannot find one for your bike.

Chombi