Every month or so I add a short blurb at the beginning of a newsletter asking for sponsors and laying out my current metrics. This always leads to multiple readers replying with interest.

The typical sponsor has gone on to book 4 slots at a time (I do not offer a bulk discount - this is just what they tend to do).

The slot is integrated right after the intro blurb and before the guest's content. I offer them a headline, some text (not too strict on character count as long as it isn't crazy) and a CTA button.