Square tube expansion plug

This is part of my Baby-Huey large format 3D printer project.
http://nerdipedia.com/tiki-index.php?page=huey

An plug which expands and jams itself into the end of a tube.

Same as my earlier one except this one is for 22mm inside dimension square tube - ie 25.4mm outside with 1.2mm wall thickness.
The nut is M8.