My girlfriend won a self inking stamp pad from Instructables for this stamped canvas bag project. They are pretty awesome mechanically: they flip the design over and get inked internally, then the pushing motion flips it back over facing the paper. But I also wanted to be able to make my own stamps.
Using TinkerCAD I extruded an SVG into 3D to about 2mm high, then put a 2mm platform underneath that design and fused those together by grouping them. The platform was about 40mm square, and it fit perfectly into this stamp pad with some double sided tape.
As you can see, the 3D printed plastic doesn't take the ink particularly well, so it doesn't make the best looking designs. I ended up taking a piece of unmounted linoleum and carving a design into that, and cutting it to 40mm on each side with scissors. Makes a better stamp, but of course it isn't totally automated!