If you saw I'd started a Tyranid Bastion with a foamboard core so I've added a template to my blog should you wish to grow your own version and need a starting point, it's here:
[url]http://40kaddict.blogspot.co.uk/2013/06/you-asked-and-great-devourer-provides.html[/url]
Updates will be following shortly on the blog and in this thread, it's going really well, here's a little further on
And the crater base with slots to fit the template: