Ok so this is a stupid question... I'm making a section of mould that is upside down (underside part of a car sill so I can't turn it around).

cheers for the reply, currently I've tried thin layers and am backing them up with further thin layers when they have turned so are soft but ridged. its far from ideal and making the process overly slow.

Can't you just jack one side of car up really high then lower it down onto a couple of oil barrels or similar (I've used ex F1 wheels before with the kit car) obviously you need to support it in a few directions, chock etc so if one support fails or the car slips it gets caught by something else but it does give access for short periods of time.

The other one I've used for access is to jack the whole car up onto high chocks like old F1 wheels to give yourself 2-3ft of clearance under

Another way would be to make a quick and dirty mould on the car and stiffen it right up so it's more of a jig than a mould, remove it and the undertray then using the stiff mould you can brace the tray off the car to ensure its the correct shape and make the propper mould. A bit long winded but