There is an interesting thread over here: (Mega Backdoor Roth In Solo 401k: Control Your Own Destiny
) about how to roll your own 401k with whatever terms you like. This is more flexible than the off the shelf offerings of solo 401ks from the major providers. You could plug in whatever "investment only" option that you like. I think that you could use whatever fund/brokerage company(s) that you like as long as they accept ownership by a trust.
I have been toying with the idea of setting one of these up without actually being self employed, (well maybe I could become an "ebay seller" business if needed) with the intent to roll existing 401k assets into it.
Of course I would not be able to make annual new contributions without 1099-misc earned income.
Depending on the size of your 401k and how much you outsource you may not end up saving anything on the fees however. YMMV.
There are probably much simpler ways to accomplish your goals, but again I was intrigued by the ability to have a completely custom plan based on hand picked providers.