Have you tried adding a border on the right hand side only of the fields? If you use this method you will need to experiment a bit with field height. It's a while since I've done it, but from memory a field height that exactly matches the body part height will work. If you want variable height, and you are not sure which field will be the biggest, just set them all to the biggest required and set them all to slide.
I guess that quiet a few people are facing this dilemma since years.
It would have been great if sliding field objects could be linked together with the option to slide to the largest object's height.
Not sure why such an obvious requirement for list printing like vertical lines in connection with sliding objects has never been implemented.
The only means of obtaining clean verticals in the body part is not using sliding fields. A huge drawback for one of my solutions, too.
Some more digging produced the following link:
You'll find a post from LaRettaK with a workaround. I checked it in FMPA 14.0.2 and it seems to work. Be aware that this workaround uses a non-documented "feature" and may brake with any new release.