There are plenty of lasercut calipers on Thingiverse.  I suppose the problem with a 3D printable caliper is getting the accuracy down.

However, why not take the accuracy out of the equation?  It should be simple enough to create a printable little caliper that has a thin slot for inserting a printable paper ruler.  The easiest way to achieve the thin slot would be to make the calipers out of two separate pieces that fit together.   Then the calipers could be designed to have a little window for viewing the sliding edge of the calipers against the paper ruler.

Yes, you could print one of the business card ones on cardstock, but why not just print a durable plastic one if you can?