Another brand is Novarossi plugs - I've got N. 5 for 65% - on the back of the glow plug package the chart (for cold plugs section) lists:
N. 5 Cold Plug
N. 6 Cold Plug
N. 7 Very Cold
N. 8 Very Cold
N. 9 Cold-Special

I havent had a chance to try the colder ones (6-9) as my used boat came w/a bunch of plugs.