I did a cludged -3S / -5S hybrid with a missile rack off the -4L/-9S warhammer (the vertical oriented one) and the RAC off the reseen Marauder (-9S/-5R model).
The missile rack needed clipping to get rid of the bit that would stand it up from the WHM's shoulder, and after filing a flat surface there I then spent a bit of time scoring some extra panel lines.
the RAC sticks out a bit from behind the Banshee, but on the whole I think it works pretty well.