Ok i am looking for a good car alarm / kill switch ideas.
So far i have been unable to find / see a good alarm for a Convertible.
I am even up to the stage of placing bolted hook ups in the garage floor .For me its all about slowing them down because we all know if they want your car they will get it.
Name bands that  i am looking at are MED with a extra speaker under the dash 95dbl in that area screeming at you it will hurt .

Help please
Ausgoat

for the most part, any of the DEI lineup is of a decent quality. Viper for example.

Depending on what you want to do you can try some different approaches. Most any sensor can be added to most any alarm system.
If you want to keep people from being able to get into the cabin area without the alarm warning them away and/or going off, you can use a 2 stage "radar" sensor.  This is the type that would have the alarm chirp when someone broke the outer perimeter that you set, and then would set the alarm off if they penetrate the inner perimeter. On a vehicle with a powered top you could even have the top put itself up when the outer perimeter is broken, but that would likely cause more problems than it would fix.

If you just want to make the car as hard as possible to be stolen, use a radar sensor to just set the alarm off as soon as anyone is in the cabin area (again you can set the size of the sphere that the sensor detects in).  Then as you say just put an alarm siren (with built in battery so it goes off even if they cut the wires & throw it) under the dash right where the would be thief would have to put their head in order to hotwire the car.

Like you said, nothing will ultimately stop a determined thief, and the options that Kevin mentioned are both very good ideas.