If you can return the "Tschako," get rid of it ASAP. I'm afraid it is haplessly wrong. That Wappen, most likely an electroformed replica, would be the correct size for an other ranks Tropenhelm.
Sorry, but the helmet is a very grotty fraud. The seam running up the front of the helmet should not there, and the side vents are the wrong style. The Feldzeichen may possibly be original.
I believe the entire helmet is fake. Critical areas to consider are:
1. The top of your helmet has the wrong shape.
2. The rear visor should have a green paper panel sewn to its underside and there should be a corresponding welt stitched to the outside edge of that visor.
3. There should be leather reinforcements (just like a Pickelhaube's) where the chinscale Rosetten attach).
It looks like the felt has been glued on. I have this exact pattern front plate in aluminum with split prongs for attaching to an enlisted Tropenhelm. As far as I know, that is the only legitimate use of this particular plate.