Solutions for external access on Sharepoint Online?

I'm looking for solutions to provide external access on Sharepoint Online, but a strategy that could work more similar to old FBA (Forms Based Authentication), where an automatizated task could generate a user/password and send to any email and work (not only MS accounts) and work fine.

In my scenario external users only have read only access on shared documents on specific Sharepoint Site, but i need segregate documents by privileges.

At now i have 3 options:

1) Inviting a user that need associate your corporate email with MS account (process not easy);

2) Include external user on my tenant (a cost problem when i talk about a large number of users)

3) I assume the operation that create a user on outlook.com and send credentials for external user.

Any idea or previous experience to help?

Everton Buzo