Another solution might be to replace the thru bolt and 2 nuts with a piece of all-thread and 4 nuts, 2 inside and 2 outside of the clamp pieces. In this way the clamping of the seat post and the clamping of the rail angles can be separated. Set the inner nuts to control the post clamping force, then jam the outer nuts against the inner ones to clamp the rail angle. Sort of like adjusting your front axle cones.

I think the spacer plate will work, though.