A “Solo-k” is a 401k retirement plan for a business with no employees except the owner and, if applicable, the owner’s spouse. The rules are the same as for a traditional 401k except that no annual reporting is required with the IRS until the balance of the plan (not account) reaches $250k. When the plan balance reaches/exceeds this limit, the employer must file an annual 5500-EZ, which is much simpler than the long-form 5500.
Moonlighting income reported on a 1099 is often used to set up a Solo-k. If you own less than 80% of your main employer, you can open the Solo-k as a second retirement account and contribute up to $56k ($62k if age 50+) in 2019. You also are not allowed to open a separate retirement account for yourself alone if you are a partner/shareholder in another business, such as a medical practice.
You must set up your plan by December 31 but you have until the due date of filing your personal tax return, including extensions, to contribute to it.