So I have a customer with a potential big project that (ofcourse) does not know what they want exactly.
The size of this project can be more that 4 or 5 months so that is a big risk. Thats why I want to sell a timebox. For me that takes away the risk of spending 10 months instead of 5 for the same price.
The problem is that I can't comeup with good arguments to convince the customer that a timebox is better for them. Any suggestions? How do you people handle this/
You could tell him that with an agile project, he gets frequent releases, so he can give feedback and update goals, even steer the project in a completely different direction than he originally envisioned.
Explain him that estimating unclear tasks is risky, so noone can do that with certainty 5 months ahead - whoever claims this is either lying to him or doesn't know what he's talking about (or actually overcharges him by overestimating manifold to be on the safe side). You may ask him if he has seen any real life products successfully delivered on time, with satisfying quality - chances are he hasn't :-) :-(
Then you may ask him whether he prefers to get a product maybe 5 months late, with functionality which fits the original specification but not what he really wants, or a potentially smaller, but working product after 5 months, which is guaranteed to do exactly what he will want then, instead of what he now thinks he wants.
Drop the buzzword - which is new to them and therefore scary - and sell a fixed time contract with explicit mention of iteration 2 feature specification, and how that would be another contract to be negotiated at a time after delivery.
Just as you have no need to tell them which text editor you use, there is no need to sell them on the time-management model you use. Timeboxing can be described to a customer in far more conventional - whence comfortable - language. You're selling a deliverable, not the means to that deliverable.