Stiff Router base

I have pretty much finished the gantry for my small router. It is a
dual ballscrew design with the ends of the gantry running on rails
that sit on two pillars. The "pillars" are 80X160mm crossection t-
slot aluminium. The gantry is a thomson superslide. Although I am
only aiming to wield a 900W Kress router *to start with* I want the
machine to be really stiff and so far I feel I have acheived that
with the gantry itself.

The next question for me is what to make the base on which the two
pillars sit on from. I intend to use t-slot panels for the actual
machining surface but these need to be attached to something and
this something must also lock the two pillars together stiffly.

The machine is around 730mmX730mm not all that big but I need to get
it or at least the base into my attic via a narrow set of spiralling
stairs, heavy solutions are out. I wondered about just using more t-
slot but this is quite expensive and bolting it together seems the
wrong way to go.

I also imagined a structure along the lines of a welded box-section
outline with a lattice of egg-box style thin alumium plate forming
interlocking webbing in the centre of the frame. This might then be
capped with two thin aluminium plates. Should be light and stiff
but perhaps it is overkill, perhaps there is a better way.

Ideas and suggestions please.

Graham