First thing to do is choose the speakers very carefully. I bought JBL GTO 937 speakers - about £40 including postage from eBay. They have a mounting depth of 75mm, and this really is the maximum you can squeeze in! If you fancy doing this, try to find some with a slightly shallower profile, even 73mm will make a difference!
First, make up some extension leads for the speaker cables - about 12" is enough. Mask the tweeter with tape to protect it from scratching while fitting.
Once you take the cabriolet door panel off, there is a hole above the speaker which was just large enough to work the new speaker into.
Before you go any further, connect the speaker wires up. Now, here's where it gets tight - as you drop the speaker down, it will struggle to get over the strengthening bar in the door. Pull the inner metal panel towards you and it will eventually drop into place. There is a ridge of folded metal around the original speaker hole, at the top there is a gap where you should be able to manipulate the protruding tweeter part through without damaging it.
The JBLs wouldn't go exactly vertical, but they are pretty close.
Next, there will be three of the four speaker mounting holes which will need to be drilled. The fourth speaker mounting hole is in fresh air unfortunately!
Once this is done, the rubber edges of the speaker will be pressing against the metalwork. I inserted some spacers into the hole and passed mounting bolts through, with washers front and back, and locking nuts to finish the job off. The spacers prevent the speaker cone from touching the metalwork.
Finally, the four screws that hold the original covers in place were cut down (otherwise they'll penetrate the new speakers), and used to refit the backing plate, then the grilles were put back in place.
And here's the finished job:
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