Edit: Once you place a horizontal line on your report, you grab one end of the line, a box will appear that you can drag around until it is vertical. They are dynamically rotatable in this regard.
What you need to do is create a vertical line on each side of your detail section. The vertical lines in the detail section will repeat with the data rows, and what it will look like is that you've got a box that grows with the data.
In the section above your detail, create a horizontal line below the data fields. Extend this line to start/stop about where your vertical lines start.
In the section below your detail, create a horizontal line above your data fields extending to where your vertical lines in the detail section stop.
You can tweak them to get them to look pretty good, but printer/OS pdf settings across computers may generate slightly different results. I would start with the vertical detail section lines, and get those to display just right, then work on the horizontals and making the ends meet to form a solid box.
The placement and selection of the lines can get a little finicky when you start putting ends of objects too close to the section dividers. Also, sometimes drawing these lines left to right or right to left or top/bottom. If copying lines doesn't look like it's working in the printed output, draw them individually.
Let me know how that works out for you.
Also, we can also develop/design custom reports for you - give our sales team a call if you are interested.