I modelled this in Cinema 4D R20 as a 'semi-retro' bike helmet.
I have uploaded the C4D as well as an FBX and an OBJ - The C4D model will have the 'native' textures and I have included one 'jpg image' for the bump.
(PS! - this model 'may' be OK for 3D printing, but I have not tried it as yet...)