2 cheap external hard drives. Use Acronis (or if you are lucky enough to have a copy of Norton Ghost) or other cloner to clone your entire hard drive including operating system to one or the other external overnight, keep the other drive in your car and swap out every day. Use a password/encryption to safeguard your data in case you lose one.
Simple, cheap, I've got this solution running in every dental or medical office I support. It not only saves their data in case of disaster, but also in case of ransomware or other serious infection.
Just so I have this straight in my head: you support multiple dental/medical offices, and every day, you drive to each office, clone their drives, and keep a copy in your car. I'm pretty much just repeating exactly what you said. Is that correct?