diagonal(diamond)shaped horn antenna-HFSS

hi all,
im trying to design an horn antenna which is fed by a diamond shaped waveguide. itried with polylines,by rotating them to 108 degrees(my requirement)and united them..but it dint work...it is showing some meshing error...i dont know y?
can anyone suggest me some ideas for this design..
also i tried with regular polyhedran,and its working..but im particular about the angle..108 degrees and the sides are equal..so its like just twisting a square with an angle 108 degrees..please suggest some ideas..

regards,
mubassira