[NTG-context] Trouble setting PDF boxes

Hi, I am having trouble setting the PDF boxes to my desired dimensions. Suppose I have \setuppapersize[*a*][*b*]. I want CropBox = MediaBox = *b*. So far, so good: I can just use cropoffset=0mm in \setuplayout. But I want TrimBox = *a*. However, as far as I can see, I can't achieve this using trimoffset in \setuplayout unless (width of *b*) – (width of *a*) = (height of *b*) – (height of *a*), which does not hold in my case. So it looks like I need to find another way to set the TrimBox (and the BleedBox, which I want to be 3mm wider and taller than the TrimBox). Does anyone know how to do this? Best, Jack