Dan
I have had several "stack packs", most of which were 2 piece, a separate section going round the mast. My present one is a one piece which has flaps round the mast which fold back and clip when open. I like this system, mine was made by Dick Batt, Batt Sails of Bosham.
Always zip from mast end, much easier than joining a zip at the boom end.
Philip
rascal, superseal 26