Somewhere around here is a "fixture" I designed/built that we used to reproduce some 1837 Greek Revival rope mould.It went all the way 'round the exterior cornice,big honkin house(R.E.Lee slept here joint).It was very "interesting" to say the least,haha.It would make the mould as long as you'd keep stuffing it into the end.
The trick was getting the pattern to reproduce itself....Huh?We got a huge shipment of dowels that were the right OD and 6' long.Then I took a std electricians "Bit"...the same size as the mould's OD.The drill got screwed into the end of a pce of dowel.The spiral on the drill sets the spacing and "times" the cutting.
After cutting a foot or so of mould....the fwd projection part of fixture gets unscrewed....elect bit removed...and then the index mechanism registers off the now started rope section.
It worked great...made real good money,cpl hundred an hr(25 years ago).We did have a little chatter on the router motor head we were using.....it would show up when we'd get greedy with feed rate/cutter getting dull.Took a 1 G. paint can of nails and hung it from the router plate using one of those black rubber cheapie straps that you see on every plumber's truck.
This was full perimeter mould.Not split like OP.