Hi Clive,
mine is about 190mm wide; its hard to measure the length due to the circle being incomplete but I suspect it may be slightly less like 180-185mm so not exactly round (and obviously the connector isn't central). The sort of triangular/coat-hanger shaped void is about 135mm at its widest and 35mm at the narrow neck; the two arms that lead to the connector are more or less on the centre line of the circle, bisecting it equally in half.
That was the element that was already in my second hand burco, so I guess it may be original and looks like your picture. I also have an unused spare one somewhere which is a different shape, like the one in
this thread, annoyingly it wasn't where I thought I'd put it, but I could try to look that out if an alternative would be useful.
EDIT: the place I bought the spare one from suggested they vary slightly in size and style (and indeed an
image search shows all sorts, some that are quite different and some that are just slightly so); may be wise not to base anything too concrete on it until you have an actual item.
Cheers
Kev