The designs on 3D-printed stamps are most often simply extruded straight upward, at an angle of 90 degrees. This makes for poor release when stamping clay. However, the letterpress plate-making process creates a comfortable draft angle automatically, this greatly assists in getting plates to release after being pressed into the clay. That means the design submitted for plate making is what is at the bottom of the recesses. Note, for example, the enclosing rectangle. It was added as containment to prevent the outside edges of the lettering from smearing outwards. That line has a stroke width of 1pt, however as noted, that width is at the bottom of the recess. The width of the line at the top is likely 3 times that. In this press of the Moabite stone the characters are about 5 mm tall. The complete enclosing rectangle is about 2 1/4" by 3". Since I am brushing pigment into the letters and cleaning the bisque with a sponge, the depth should not be too much less than this (K152).
We find the 0.047 relief depth shown here is best (K152). Shallower ones will stamp a crisper design but K152 is better if pigment will be used to highlight the recesses. For some things it can be valuable to put border around the outside of a design so that when the stamp is pressed hard into the clay, the edges do not smear outward. These do not actually need to be stuck to a piece of wood, just lay them face down on the clay and use a wooden block to press them. Because they are flexible it is easy to peel them out. When the clay is stiff enough no parting agent is needed. The cost: In 2022 the minimum charge is $35 for about 50 square inches. They accept PDF and bit image files and the shopping cart enables previewing. The cart might generate CMYK plates (four of them for process color printing), just remove the CMY ones and keep the K (black). The most common mistake is having too much detail or too small printing. Or forgetting to make them reverse-reading. It is best to make your images using vector graphic software like Illustrator or Inkscape.
Boxcar Press website