Very thin compared to “Jade Bath SPC Niches”, available on WF, but they’re too wide for this application, (trying to use the narrow stud spaces for a bath water filtration unit).
This brand is double the price and is barely strong enough, needing accurate wall framing surrounding it. It doesn’t flex, but I think it is likely to be brittle, although it is stated to be ABS. It is not cracked, (I checked),so it will be waterproof as long as it is not struck with a hammer or screws overtightened. It is a challenge to get marble thin enough to retain the interior space.
The interior dimensions are 5-34” x 13-58”, at the front edge, FYI to anyone interested.