Here are bq Base enlargement kit on my Prusa i3 Hephestos.
Larger print area: 300x200mm
I have used: Y Carriage belt holder for Prusa i3:
http://www.thingiverse.com/thing:619347 instead of the original.
I added four simple feet which I have drawn in SolidWorks, to lift it all with a few mm.
So Aluminium Base don't touch. It gave more stability.