The vehicle construction rules are finally in the Traveller SRD and (thus) now Open Content, allowing you to create and publish your own vehicle designs and Modifications!
It is part of the Traveller Developer's Pack, which can be downloaded from;
http://www.mongoosepublishing.com/pdf/travdevpack.zip
It is part of the Traveller Developer's Pack, which can be downloaded from;
http://www.mongoosepublishing.com/pdf/travdevpack.zip