I let my soon loose on TinkerCad, alone and unsupervised, and this is what he came up with.
So when his good behavior earned him a 3D print of whatever he wanted, naturally what did he choose? Now at it's natural scale this is the biggest single thing I've printed to date. But I'm really trusting my new firmware, so what the heck.netfabb to fix the geometry and we're ready to try again.
The files are available for download on Thingiverse.