Solo 401k for self employed

If a self employed individual - operating as a SMLLC (disregarded entity) - opens a solo 401k: does the 401k need to be in the name of the LLC? Or in the individual’s name?

I'm gonna say the LLC's, it's a company plan. Did it have to be open by 12/31?

I thought that rule changed to say open by filing of return.

Section 201 of the SECURE Act changed the deadline for establishing a plan but contrary to how I interpreted it in another conversation, it doesn't apply to 401k plans due to timing of the elective deferral. According to Ed Slott:

The SECURE Act change brings the deadline for establishing new qualified plans in line with what the deadline has been for setting up new SEP IRAs.

Importantly, the new extended deadline only applies to qualified plans that are entirely employer funded, such as profit sharing plans and pension plans. It does not apply to 401(k) plans. Because of the timing rules for 401(k) deferral elections, a new 401(k) plan effective in one year cannot be established after the end of that year.

And from another site (mysolo401k.net):

While the 2019 SECURE Act extended the deadline to adopt a Solo 401k plan from the end of the year to the business tax return deadline including any timely filed extension, the IRS has not yet updated its prior guidelines stating that employee contributions must be elected by the end of the year.

Therefore until the IRS issues final guidance, the conservative approach is to adopt the plan by 12/31/2020 to ensure that both employee and employer contributions can be made for 2020 in 2021 by your business tax return plus extension.

A survey of plan sponsors seems to confirm this as they require a solo 401k to be established by 12/31.

I'll also edit the other conversation.