This model takes the essence of previouse stamping jigs like Blockmit's but improves them and makes it ultra 3d printable:
a little notch on every part provides a place to put seams in. Make sure your slicer's setting for “Z Seam Alignment” is set to “Sharpest Corner”. This makes sure, that no seam prevents the parts from fitting
the bottom is chamfered to minimize the effect of elephant's foot
the middle part of the jig is extra long and sturdy, so the metal stamp has more support and can't tilt as much
the outer part can be easily and securely hold thanks to the big fillet
The uploaded STL is made for ISO 7093 M8 washers and quadratic 6 mm punch stamps. If you need somehting different you can modify the model through variables in onshape.