Me and this other guy already tried, there is a topic in the modification section but I can't seem to remember the name but its there.
These will likely need to be bot based vehicles, or your just going to have a bad time.
Why can it only be bot-based vehicles?
UPDATE: newspeed12 (an affiliate of mine) is learning how to use a modeling program.
I gave him a request to do the simplest of the vehicles here. If he fails to complete it I will be fine with it since he's a beginner.