My rotisserie for neck through worked partly because PB ES style basses are relatively light. Also, there are a couple of cinder blocks holding down one side of the jig to counterbalance the bass. Not sure that's visible in the pics.

The other thing that I found is that I needed a way to lock the rotisserie in place. Not much of an issue when dealing with a body attached to a pole where the neck goes. Much more important when a whole guitar/bass is attached via a pickup cavity. Particularly if you are like me and want to be able spin it to catch drips ;-)