As a freelancer with no employer plan, which solo 401(k) provider offers the easiest setup and lowest fees for index-fund investing?

We ask the same underlying question in different ways.

• As a freelancer with no employer plan, which solo 401(k) provider offers the easiest setup and lowest fees for index-fund investing?
• I am a freelancer opening a solo 401(k) and want to know the most efficient way to put my money into low-cost index funds.
• I am a freelancer opening a solo 401(k) and want to know how to put my money into inexpensive index funds.