For this purpose, a Solo 401K is just like any other 401K: You can also contribute to a "regular" IRA (or Roth IRA, etc). At least I hope so, because I've done that in the past!
Here's a TurboTax online help answer that agrees with this:
Link to IRA and Solo 401K answer
Obviously, the regular income phaseouts for the IRA deduction still apply.
Be careful out there. You are just 10 years away from having to take RMDs from these tax-deferred accounts, so look ahead and see what that means to you in the future regarding tax brackets, etc. Everything you put into these accounts, and their earnings, will be taxed at your regular income rate rather than the (possibly far more favorable) LTCG and Dividend/Interest income rates if the money had been kept in "regular" after-tax accounts. Much depends on your present and future anticipated tax brackets. In my case it makes sense to use the Solo401K and tIRA contributions to get our present taxable income into the same bracket we'll probably see in retirement, but not much below that.