This might be better.
http://www.freecadweb.org/
It has a fairly generic 3D modelling layout and on first use it seems quite simple to use. It took me a few minutes to find the dimensioning tools, but once found (FIX commands on the Constraints toolbar in the Sketch environment) it was pretty quick & easy to extrude a simple 3D shape and add some detail. Well worth a go I'd say. I'm going to leave it on my astronomy laptop.