First the State needs to define what constitutes a "road legal vehicle"? Does a spare tire, or trunk define a road legal vehicle? What about a radio and A/C? Maybe it's the head lights, brake lights, tail lights and blinkers that define something as "a road legal vehicle"? Lay a cabinet door across the front and one across the back, remember to leave the hinges in tact! There, it has doors, 2 of them! Strap one across the roof. Three!, is the state appeased now? Did Indiana ever tag the BMW Isetta?(look it up, it's funky but has 4 wheels!) I like the bits of Plexiglas idea! Does the State issue license plates for the Amish buggies? (Remember kids, a bureaucracy only exists to insure that the bureaucracy has a job tomorrow!) Have the State legally define "doors" so you can be sure to meet the expectations! If the state can't define "door" legally, the state can not then enforce "door" legally. The state can't CONVICT a person of "assault" if the victim is a mannequin, this person may be a sicko, but assault it ain't!; (unless the defendant's last name is Trump.) If the State can't define what "a woman" is, then we'll no longer have any need for "Women's Prisons"! (See?, we're saving money already!) *** Relevant movie moment: Cop: "Sir?, Are you classified as Human?" Corbin Dallas: "No, I am a meat popsicle!"(5th Element)*** How is the state tagging Tesla's?, they have no engine, just motors. Maybe you need a lawyer? Or Montana.