Interconnection by square waveguide

Hi all,

I have two fixed square waveguides with slightly different cross section, and they are shifted along x and y axis.
I have to design (in HFSS) a waveguide interconnection that allow this connection and implement the x-y shift.
I have just tried to use mitered and continuous bands (with a single step for the cross section) but the Return Loss is too low.
Could you help me.
Valuable suggestions and help would be much better and appreciable.

Marco