The deadline for establishing a solo 401(k) plan for 2020 has not yet passed. Due to the changes in the tax code in the SECURE Act of 2019, the deadline for establishing a solo 401(k) is now the due date of your tax return; IRS Pub 560 has not yet been updated for 2020 to reflect that change. However, you will be limited to just the employer contribution for 2020 since the deadline for electing to make an elective deferral is still year-end. The maximum employer contribution to the solo 401(k) would be the same as the maximum contribution to a SEP plan (which is also an employer contribution).
By contributing to a workplace retirement plan (either a SEP plan or a solo 401(k)), you are considered to be an active participant in a workplace retirement plan and whether or not you will be eligible to deduct a traditional IRA contribution will depend on your filing status and your modified AGI for the purpose. TurboTax will automatically determine the permissible deduction when you enter a traditional IRA contribution.
https://www.irs.gov/retirement-plans/plan-participant-employee/2020-ira-contribution-and-deduction-l...
