Ask HNL: Private credit line logistics

Let's say a semi-wealthy benefactor wants to provide my company with a revolving private line of credit for some relatively small amount--let's say $50k. I do mean private in the sense of a private individual, strictly peer to-peer; for a variety of reasons, commercial lending and credit line products are outside the scope of this question.

From a purely logistical banking perspective, what is the simplest way to structure this arrangement? Are there third-party companies or escrow agents that specialise in this?

I'm not interested in a joint or cosigned deposit account. This needs to be reasonably arms-length in that sense at least.

My initial thought was to set up a separate deposit account for expenses and have him wire/ACH the $50k into it, and at the end of the month I have to return the $50k to him + interest. But then it's not really revolving; I have to pay in way interest on the full $50k because he's out $50k in cash, even if I only need, say, $35k that month. That cuts out a key premise of this instrument. And besides, we get eaten alive by transaction fees (even ACH) sending all this back and forth all the time, even if he agrees to keep the principal deposited and only be disbursed the interest.

What's a better way? Can the investor just walk into the bank, hand them $50k and have them issue me a credit line and intermediate it? Can he just guarantee a retail credit line product as a cosigner and be paid (by me) some extra interest on top of what the bank charges? Both of us would rather avoid that approach because the bank would want too much interest; it's best that he make the money and that this be a peer-to-peer credit line in the truest sense. I understand that some transaction fees are involved no matter how it's done; if it's going to be electronically managed, someone's going to get their cut. But I don't want it to be a 5-15% APR cut.

If you have any particularly ingenious tips on how to structure this, feedback is much-appreciated. As you can probably guess, I have operated entirely bootstrapped, from cash and have no experience with the mechanics of acquiring external capitalisation of any kind, however hypothetical.

I'm considering asking someone for such a thing, but before I do that I'd like to know how it is actually done from a pedestrian angle.