I have seen 2 ways to do it. I think Death Touch screwed a PVC T into the top of the head at the right location and angle so that when it was put on a PVC 'neck' it looked the way you wanted. I wanted to seal the head so I could put some light in it, so I cut out a scrap of plywood that would fit the cauldron opening, attached 3 paperclips so they would stick out from the edges of the plywood as hooks which would hold the head. Then I mounted the plywood to the top of my scarecrow post at the proper angle (sorry no pictures). You can see the difference in the following pictures, the first is without the base sealed, the second is with (look at the eyes).
As far as rain is concerned, myself and others have had mache props out in the weather, and they may soften up a little but have always hardened back up when brought in. The key is to seal the mache well with polyeurathane. In Central Florida the only way I could get mache to stay hard in the humidity was to seal everything well.