Admin Page Wireframe Tools?

Has anyone got a wireframe tool for the WordPress Admin page? I would like to be able to mockup some ideas for a plugin and can’t find a UI Wireframe template anywhere.

Ideally I would love to get an Illustrator file of the admin interface, but I would take just about anything vector-based.

If something like this doesn’t exist, why? I did find one mockup template from six years ago–not a lot of help and it was a Photoshop document.

Any help would be greatly appreciated.