HELP! Trying to add thickness to a model!

I am a BEGINNER to sketch up and I am trying to add thickness to a model that I got from the 3D warehouse so I can 3D print it. I tried using the Fredo joint push/pull plugin but it only adds thickness to the sides of the model, not the face. Please help!

Thanks in advance,

P.S. If you don’t mind could you send me the file with the thickness if you could do it. (I would like a uniform thickness of 1 cm on a 3 parts). Thanks.

Solid Head 4 (Head height is 20cm).skp (674.7 KB)