Yes. One of the significant benefits of a solo 401k is that it does not require the participant to hire a bank or trust company to serve as trustee or custodian. The plan participant is allowed to serve as trustee. Accordingly, the plan assets are under the authority of the solo 401k participant.
Another benefit is that this allows a solo 401k to eliminate the expense and delays associated with an IRA custodian. You can act quickly when the right investment comes along.