Such boxes are on the market from Rule and Attwood and probably a few more. Have heard that they need a lot of cleaning maintenance and break quite often.
Also, they are not a holding tank, the capacity is small so that you are forced to pump out when showering or using the sink.
Najad's solution to this is that you pump out the shower direct, without an intermediate tank, via a small diaphrame pump. In this case a Jabsco 37202, which can run dry.
I have extended on that solution and had them put in a Y-valve between shower sump and wash basin, so that I now have a pump for each heads compartment which can selectively pump the basin or the shower and two big holes and seacocks less in the hull.
Both have a strainer that I need to clean about once every 4 weeks of usage. Not a big deal. If I had to retain the grey water, it would be easy to divert the output to one of the holding tanks.
We have the Y-valves usually set to drain the washing basins and when somebody uses the shower, he turns it to shower temporarely and back to the basin. If the pump is not running, the basin fills without having to close the drain, which is quite convenient. I'm very happy with that solution, it is clean and it works well.