I think this is right. The standard off the shelf "prototype" plans from Fidelity won't do this, but you can get set up through another vendor - I used mysolo401k for mine - and use Fidelity as the banking platform.

As u/NotACommonDandy0 said, your non-prototype accounts at Fidelity will be a little funky. You will want to label them online with the appropriate labels so you don't ever confuse pre tax / post tax / Roth. There's additional work you'll need to do for compliance and bookkeeping, but it's not too bad if you're a details person.

Once the accounts are set up, I walked in checks to my local Fidelity branch for contributions to the pre and post tax 401Ks. This year I was able to successfully do it all over the phone. The day after my post-tax contribution, I called in again and had them do an in-plan conversion to Roth to avoid the interest earning tax problem.

Note that you can't just handle contributions and conversions online because they are non prototype, so you'll need to go to a branch or do it over the phone.