For the last 20 years I have used a heavy vinyl luggage garment bag with full zippers (turned it sideways & added straps for handles), adding sheets of bubble pack inside for a soft lining.
Prior to that I wrapped the keyboard's original cardboard box entirely with clear shipping tape, created an opening on the side of the box, so I could slide the keyboard in, and installed a handle system (made from those plastic handles you can find on larger appliance or TV boxes). I kept and used the styrofoam liners for the padding.
I only go and play gigs part-time though, so if you are travelling with your keyboard a lot it would be worth investing in a proper case or bag.