I just converted to an old school seatpost to solve a setback issue with my B17 saddle. BMX seatposts are the answer.

Try:
Redline Laid Back Seatpost (25.4)
Problem Solvers Shim (25.4 to your seatpost size e.g. 27.2)
SBS Seat Rail Clamp

Ran me about $25 using Niagara via Amazon. I had to hack off a couple inches off the bottom of the seatpost for it to work with my bike. There are expensive rail clamps that have finer angle adjustment (brompton pentaclip) if you need finer adjustment than the cheapie clamp.