This is the sort of wrench I have in mind:
In the Wikipedia article (Socket Wrench) displaying this image, it is described as a spark plug wrench, but there are wrenches of this style in many other sizes, and for many other purposes – anywhere one has no access from the side and a long bolt or stem preventing even deep 'conventional' sockets (having a square socket for a drive on the other end) reaching the nut.
The image itself is labeled "box spanner", but if I search for that at Home Depot, Harbor Freight, etc., I get back the type of wrench that has a closed, 12-point ring on the end of a handle, and if I search for "spark plug wrench", I get tools specifically for that purpose. I suspect that "box spanner" is British usage; is there a term in American usage that picks out just this style of wrench, regardless of its purpose?
Update: Thanks to everyone for your help and advice. As it happens, I was able to make a suitable tool by cutting six notches in the end of a steel pipe. While this was not as good as a real wrench, it was sufficient to grab the points of the nut and stop it from turning, in the manner of a basket strainer wrench (as suggeseted by Jimmy James.).
Best Answer
This is a tubular wrench.
Apart from the British "box wrench" term, "tubular" is in my experience the most common generic name for these tools. You can get a full set of them (I have one, though in metric sizes).
It looks like googling "tubular box wrench" is the best way to get the right stuff while eliminating the American meaning of "box wrench".