Polyhedron shape with two bases

I am trying to create a Polyhedron of length 50 cm and with two bases of dimensions 20 x 20 cm2 and 20 x 10 cm2. I tried to follow the recipe given in manual however with no success. Any help on how one should define G4Polyhedron for such shape will be highly appreciable. Thank you in advance.

Such solid in the Geant4 terminology is called trapezoid, please look at G4Trap and G4Trd: