Scooter used to bang shit together outta sheetmetal and weld, then fiberglass & bondo when it needed to be pretty. probably cheapest and most accesible route,
2nd would be molding outta cheap wood and cutting/gluing a form, then fiberglassing it.
3d printing is cool and kinda pricier and specialized but most colleges have community access equipment at low cost. Getting the dimensions right is a lot harder without a physical duplicate though. like you can't design on computer and mock it up at the same time so it'd probably take some serious trial and error.
also theyre a little big for most avoilable units, even a 1 foot cub printer is kinda on the larger side and most covers are longer than that diagonal