Well, I'm all about my SI Turbomatic Gel Flow. They're not cheap and last I checked they weren't in SI's trial program, but if you can find one to try, it's I think as T-shaped as it gets in a 153 mm width, and the cut-out is very generous (both in width and front-to-back) thanks to the domed shape.

Shifting back, but not being able to stay back, says one of two things to me (possibly both) - your cockpit is too long, and/or your saddle is too wedge (pear) shaped.

What saddle are you riding now - what else have you tried?