The announcement thread for PGS constraints included a link to an open-source place which had a vehicle made from PGS constraints. You should be able to use it as a reference to create any type of land vehicle with PGS constraints.
This is understandably frustrating, but if you do not have access to a category, please do not attempt to circumvent that restriction by posting the same thread in another category. This thread is fine because it doubles as a request for tips on making constraint vehicles, but you should modify the post so that it isn’t requesting a feature.
The trial program is an evaluation period – users are intentionally limited in what they can post for evaluation purposes. This is only a temporary program, so as long as you actively and constructively contribute to the categories you do have access to, you’ll move on to be a full member with access to every category.