[eki912]

Are you sure all fuses are out ?
Do Boxsters have 2 fuse boards ?
Anyway, if you have that meter , and disconnect the + cable , and put that meter between battery + and your + cable shoe, and measure AMPS, you should get a reading of current consumed at that specific moment.
It shouldn't be much.Naturally, something will go through all the time because of the alarm etc.
When did this start to happen ? Any mods done to the car ( stereos,amplifiers etc ) ???
If you don't know how to use the meter, don't fool around too much. Go to a professional.As these cars are full of electronics you can do some harm easily, if something goes wrong.
So be careful.

Edit : Has the car been in an accident lately ? A work colleague few years ago had a small collision.After the repairs were done,all seemed OK.Except the battery, drained always, but lasted for a few days.We used the same method to trace down the fault.With the meter in place, we saw that there was a constant draw of current, altough everything was off.Started taking out fuses, one by one,putting them back immediately,finally reaching a fuse that made that needle drop to 0 amps.Then simply checked what fuse it was.It was the immobiliser,slightly damaged in that small collision.Maybe it was a wire insulation, ever so lightly broken ? Or something like that. Eventually, if I recall correctly the whole module was replaced and problem disappeared.
In your case it would be helpful to know how much current a Boxster uses when all is off and so forth.Then compare it to the reading on your car.
Generally, if your battery is flat in a few hours, you have a BIG current leak....or a really bad battery.But if your battery holds charge when disconnected, it should be OK.
Have you checked the cabin lamps? Trunk lamp? Engine bay lamp ( is there one like that in a Boxster ) ?