My 1984 PKN10 with 24mm seatpost has this very same problem, so I found that I had to lengthen the slot downward in order to allow sufficient clamping force to be generated with a standard binder bolt.
I lengthened the slot a few mm and had to go a few more mm after that, but finally no more slippage occurs now!

I used a carbide cutter in my Dremel Moto Tool, didn't do a very straight cut and yeah this photo isn't great either, but you can see how long that the slot is.