[Gary in WA]

Because the pan is not move-able, due to plumbing, you could add material, as Bob said to get it into the pan. At that time, you should shim the wall to make it square (or match the pan).

Is your can light rated suitable for wet location, with rubber/foam gasket under trim ring? It looks like a standard can to me, but my eyes have been wrong before! Be safe, GBAR