Like many others I'm making an oversized stalk-around type costume built on a backpack/PVC frame with one of the giant foam skulls as a head that will move when I move my head via an attached helmet. I came up with what I think is a unique solution for the universal joint needed at the neck to allow the skull to move freely.
Following posts on various forums I was looking into using a swivel tripod head or a swivel speaker mount but both of those would cost more money than I wanted to spend. I had a brainstorm one night and remembered our Swiffer floor sweeper had a universal joint at the end of the handle to allow it to move freely around furniture when cleaning the floor. I grabbed it out of the closet and had a look. With some modification it would work perfect. First step was to sneak it past my wife and out to the garage. Actually we also had a knock-off version from the dollar store that was similar so I used that. Then I sawed off the ends using my chop saw so it would better fit the skull and glued the modified Swiffer to a scrap of 1" foam. I then used an xacto knife to cut out a circle in the bottom of the skull with enough clearance for the universal joint to move freely. The scrap of foam was cut to fit and glued into the bottom of the skull. I'll use an old cut-off wooden broom handle and a T-Joint to connect it to the PVC frame.
Here's some in-progress photos...