Working Capital in exchange for equity

How would you structure an equity transaction where large portions of the uses will increase working capital?
A lot of asset-light B2B service business owners want to sell in a few years but are struggling with working capital now. What would you offer to get a foot in the door?

I can think of the following options and would like to hear your feedback or further input:
- Convertible note
- Preferred equity (with voting rights)
- Call option (in or at the money?)

Thank you.