I have to ask, how much standing water sits in a niche once it is tiled and grouted? John's example was after sitting for many hours...with the tile and grout, you might get a little dampness part way into the grout, but by the next day, it's dried out, and probably way before that. So, there'd be no liquid water to seep through the thing. If you're really worried about it, it would take you all of a couple of minutes to eliminate that possibility entirely. If you install it properly, neither the surface nor the foam of it is damaged by moisture. (The foam is extruded polystyrene, AKA styrofoam, the same material as the common coffee cup, but much denser.) The odds of any manufacture's premade niches meeting your exact size requirement, though, isn't all that high. There's at least a handful of companies that make niches, and making your own isn't all that hard, lots of choices.
It's nice, but not essential, to minimize the wall tile cuttings to make the niche fit, as is the visual impact of minimizing cuts inside. It's all about lining the grout lines up, and the visual impact...but, that has nothing to do with whether it will function.