But in your case, if the HR department or literally the company knows your work for more than 12 months, HR team can sign it on behalf of the CEO.
I am not sure what content you have inside, but if I am you, I will make it easiest and quickest for you, CEO, and the HR person. One idea I have is to have the CEO use his company email to saying (or cc-ing) HR team to sign on behalf of the CEO. While you can print the email, and add it as the appendix of the LOR before the HR sign it. This way may ensure the smooth and comfortable on all sides.