Matt,
It's probably more like 65x65mm.
That's from memory.
Thought I had one lying around, had a quick look but couldn't find it.
The current systen is a plate 80mm wide starting around 33mm behind the centre of the cones and wrapping around the front of the beam with a 19x19 sq hollow section around 25 long welded to the front on its side to take a spinnaker pole.
Pole slips over this and a pin goes through it to prenet it from twisting as the spinnaker is pulled in and out of the mid pole hoop.
Actually the latest system is a little more complicated but what I described will work.

Regards,
Phill

I know that the voices in my head aint real,
but they have some pretty good ideas.
There is no such thing as a quick fix and I've never had free lunch!