Winter is high season. Not sure you're going to find a charter company to give you a deal for a 4 to 6 week charter when they're all hoping to charter the boat at it's highest rates. So you might really be looking for a boat that is not in a charter fleet. They surely exist but I don't know where to point you. Keep in mind that the number of boats is in pretty short supply right now. That might improve a bit by next winter but it's still going to be a smaller fleet that last year. And likely demand is going to be better than it has been this year. You might find it way easier to charter a month time in the late spring to early summer.
I wish you luck!