[aerohead]

The leading edge of the trailer and gap-filler connection point, optimum-wise, would be matched, and both would have a smaller cross-section than at the beginning of the gap-filler, respecting the elevational and plan-view taper of a streamline profile.
Since all the aft-body is immersed in an increasing-pressure gradient, and all the air would like to flow 'forwards' if given the chance, the only thing keeping this from happening is the streamline profile. This is its 'job.'
So we've got to be especially careful about any pressure 'spikes' along the pathway. The 'smoother' the better.
Some 'accordion' bellows flow perturbation might be tolerated. Expensive wind tunnel, or CFD testing would verify that or not.