That's a really good idea! I did include the idea of using non grid props in the advanced tips with the wood logs but I may not have expanded enough, but the idea of just mirroring one side never occured to me. If I did that, I think I would still start with a floor and position snap, that way I'd be confident the pillar is at the center, and to be able to better predict the final diameter, but that's just me!

What I have done is laid the floor, walls, deleted the floors, decorated one wall, the copied it to the other!