I'd like to make a mould of some fibreglass racing car parts, in particular the side pods of a single seater.  The original parts are flex so I'm concerned that if I make a mould then it might not be straight.

If a split mould is required and you can't get the whole thing done on the car, then make the main section of the mould and then remove it from the car and rely on the mould to hold the part in the correct position. Beware of high slip release agents and pre-release if you take this approach.