Well, this isn't quite as good as one of Tamar's posts but I thought I'd make my contribution
These techniques are likely to be applicable to a number of Scalextric or Carrera shells with potential for use in GT3 racing. In particular this is relevant to the DiSCA GT3 Euroseries regulations as all of the mods here are allowed within those regulations.
The finished shell will of course require a polycarbonate interior with driver's head, bust, seat back and steering wheel.
Currently my Bentley body tips the scale at 19.2g, adding ~1.5g for the interior I should come in at less than 21.0g. That's still less than the factory issue bare shell by Scalextric, so there are gains to be had!
You could of course go to town, but the gains made vs. risk of fragility are not worth it in my opinion. I'm satisfied that this shell will be competitive on a grid requiring a 19.0g minimum body mass, so let's see how it goes in Manchester!