Agree on a trust fund for the plan patrimony – The assets of a plan must be held in trust to ensure that the estate is used exclusively for the benefit of the participants and their beneficiaries. The trust must have at least one agent to manage contributions, plan investments and distributions to the 401(k) plan. Since the financial integrity of the plan depends on the trustee, this is one of the most important decisions you will make when developing a 401(k) plan. If you are implementing your insurance contract plan, the contracts should not be held in trust. 401k solo plans, like all qualified pension plans, must be written. The written planning requirement means that a 401k plan must be included in a formal planning document.