Making My Own Dungeon World Tiled Dungeon TableTop

[O Dinas Powys]

is this the kind of thing you are thinking of....i know this is water caverns but you could use a similar principle:
http://www.hirstarts.com/cavern/watercavern.html

Not really, I was thinking far more conservatively    However, if something like that was to become available... 

My idea was the current floor without walls in a few sizes (from 2x2 up to HUGE) with the current walls as separate edging.

Three types of edge: open ended, inward corner on one end and outward corner on one end, say multiples of 3-squares long with a 1 square strip of floor sections for stability.

With these bits you should be able to construct irregular rooms and surround them in a wall to fit in with the available dungeon pieces, which would help lock in the wall segments.