iPhone 5 case - Cello

I wanted a cello on my iPhone case.

I got the case STEP file from GrabCAD at https://grabcad.com/library/basic-iphone-5-5s-case-design-1
I would have used this iPhone case (http://www.thingiverse.com/thing:36903) but for some reason the STEP model wouldn't appear in 123D Design.

The cello sketch is from http://www.123dapp.com/123D_Design/Cello/4322696 which I just extruded down into the case to subtract.

I'm still very inexperienced in 3D modeling, so thank you to the people above for