Time for a confession: I've been working on an Epic Necron army! Just because there are no official models doesn't mean it can't be done: there are a number of miniatures from other manufacturers that work excellently as proxies.
Necrons don't use Titans per se, the closest thing to a Titan on the Necron Epic army list (a fan effort) is the Tomb Stalker, which is described as an oversized Tomb Spider. This makes it very easy to create one: all you do is take a 40k scale Tomb Spider and mess around with it a bit:
One of the things I've learned from painting this miniature is how to produce "seamless" decals: I've had some very bad experiences with decals in the past which has put me off using them. I decided to bite the bullet this time and am very pleased with the technique I came up with (which is probably worth a post of its own).
I have a few more miniatures to show you from my epic Necron army: keep 'em peeled!