I have tried a few of the different methods now and they all work well. My latest attempt I was able to get straight lines for the sides and a very nice carve on the top. However, some of my lines were not precise enough and I ended up with a few gaps. Hopefully I will have some time tonight to have another go at it.

What rendering program are you using? I have been using Bryce 7 but think I may need to try out some other renderers.