I have all the materials to craft Mjolnir now - to the Forge! ... for real this time :)
As I'm still lacking a hot wire cutter I've decided to try the Pepakura method instead. Pepakura is paper modeling. Using the Pepakura Designer software you can take a pre-made paper folding file (such as the Mjolnir one here: http://www.lacasadeel.net/2012/03/fabricando-el-martillo-de-thor-en-papercraft.html) and edit it following this YouTube video in order to make a foam version.
So now I have these materials to play with:
Cutting out the template onto the foam was easy and then using wood glue I've started to form the hammer head:
By Thor's Beard!
I've started growing the beard but it is currently unimpressive: