Anyone used Fluval 15442 nano stand before?

I have never used one.

But, from seeing the stand, I'd only ever consider using it vertically.. I don't recall it being perfectly flat along the sides.

12g should not be overly heavy, but if it's a similar tank to the one I have, (The Mr Aqua one) I believe it's very sensitive to the surface it's set on.

Personally, I would say that a piece of plywood, cut to an appropriate size, then stained or whatever would be much better. But it's personal opinion.
 
AquariaCentral.com