Personally, I like using the Spinfit CP145. They're not wide-bore, but the way they're shaped fires the high frequencies a good deal on the U12t. I feel more comfortable using them than the stock silicon tips which allow the nozzle to get really close-up to your ear. On the other hand, I've found the Final E tips to neuter the highs - I used to think the U12t was a darker IEM! - which might a good option if you're sensitive to the U12t's treble peak.
As for foams, I don't really use them, but the stock ones are nice if you want a smoother treble response. They tame some of the 15kHz peak; of course, you get less air as a result.
The modules also make small differences to the U12t outside of the bass; the M15 exhibits more air and opens up the stage ever-so-slightly more to my ears. Of course, that could be more of a perceptual shift due to there simply being less bass.