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Its fairly easy enough to setup a Solo 401k. While it could be difficult to send contributions to it directly, you can contribute to an IRA and then rollover into the Solo 401k.

Solo 401Ks are also useful for enabling backdoor roth IRA contributions. For those with traditional 401Ks, one doesn't want to roll them over into a traditional IRA as that would remove one's ability to do backdoor roths.