Thanks, guys, for your assistance here. The connector I was thinking about is indeed 2x8 - 16 pins - it's just that the pic was for a different size. Not sure I'll play with FreeCAD yet -- there are too many other projects on the go -- but good pointer anyway.
I was thinking that the latch arms would hang over the board edge -- don't want to waste the board space -- but worth remembering that stuff anyway.
I had a look at the 3d view of my current effort last night and it was without a PCB... components all floating in space. I guess I've left something out of my design, but what?
Finally, my board will be a Beaglebone cape, so I have used a schematic that includes 'pins' for the expansion connectors and built a 'footprint' for them, with marks for the board edge and mounting holes to align to. This footprint uses footprint pads to make the holes with the right names and act as the physical side of the schema pins.
I do need an actual pin header too, though, which of course has its own pads, and so of course the two sets of pads conflict. The only thing I can think of doing is make a new footprint for the connector which doesn't actually have any pads. It seems icky, but is that the best option?
Many thanks again,