Do you need to be reasonable with CAD if you want to use one? I know CorelDraw but only 2D.
I have always been dreadful with 3D editing tools but I am really pleased with TinkerCAD, which is a free web based editor from AutoCAD. You drag basic shapes onto a workspace and modify and combine them. Any shape can be made into a negative, which can be grouped with and object to make spaces.
So it is a sort of 3D version of object based drawing packages. If you are not doing serious engineering it is worth a try.
