do not try and create vehicle outlines. you will be there for ever and never get them as accurate as the ones available. the main reason digitised vehicle outline software is not 100% accurate is because we digitise a flat picture and cannot compensate for curves or 3D shapes of a vehicle.
i recon Impacts Software is the most accurate outline library available, its the software we use daily and they offer our UKSG group members discount, so ticks all our boxes. 😀
https://www.impactgs.co.uk
an alternative method is to buy a cheap tripod from the likes of Jessops, (£20)
take a side on, centred horizontally and vertically positioned Picture.
measure the door handle on the side and scale the picture up till the on screen handle is exact size it is in real life and your ready to go.
everything you design on this scaled picture within your sign software will now scale accordingly. However, you will need to do this with front and back and every other vehicle you come across. which is why ide say revert to the outline software disks.