This is the biggest daggerboard type monohull I know of. Since it didn't come with a daggerboard and I don't know anything usefull about the manufacture (only that it's Italian). I need to figure out what the daggerboard is supposed to look like.
I took dimensions from the daggerboard trunk top and bottom to estimate the taper and profile of this monster. I can't immagine anything taking more than a five foot daggerboard, so I'll start with that length.
I mocked up a daggerboard from old bits of wood I had kicking around. This is BIG (a bit heavy too).

I intend to try it with this and cut off some of the length a bit at a time until the performance suffers. Then I'll know how short is too short, then I can make a permanent daggerboard a bit longer than that.