TVR guys also drill the holes.
My Tasmin came with circular holes already in the floor "To let the water out"
A friend bought a new M2 Elise and when he drove around corners, he could hear someone rattling from one side of the car to the other.
The dealer worked out it was a rivet head that had come off inside the tub. They first tried to trap it by drilling holes on either side and then pumping sticky glue foam into the tub (then drove it around trying to get the piece to stick in the foam). Unfortunately this didn't work!
The next plan was to cut more holes, and larger ones!, in the bottom and drive around hoping it would fall out and then patch the holes.
This worked! However, when he saw his car, which now resembled a large red, car-shaped, piece of Swiss Cheese, he told them they could keep it and replace the car.
Sorry, the drilling holes reminded me of this car story.
...talk about mistakes you don't want to make! 