Well... First point is that ALL computer images, in this contexts therefore the maps are rectangular. So this is a basic limitation from the texture side. Second is, that you simply need to make parts invisible in any way. By clipping or by transparency. And a humorous remark maybe. We maybe PLAY god with programs like this sometimes but we ARE NOT! So in difference to him (or her?) we have to deal with limitations!

One day your ship comes in - but you're at the airport.