Soft jaws to suit a 4" (100mm) vice. The internal dimensions are 102mm wide, 20mm high, 10mm deep. There's a horizontal slot and vertical slot included. After printing I made a version 2 (also included here) with 3 vertical slots.

I printed them with the face of the jaws on the bed, without support.

Update: added a new version with holes to suit 8x1mm magnets. The holes include a small amount of clearance (8.4mm radius x 1.2mm depth) for ease of fitting.

More recently, I created a 3" version of these here (with some slight tweaks to the design): https://www.thingiverse.com/thing:4832335