The goal of this project was to have have a keycap design which separates the supporting piece (which attaches to the switch) from the top of the key (the actual design on show). There are a few reasons for this:

• It allows the cap to be printed without supports, which are a pain to get out of the bottom
• You can switch out the design part without having to reprint the entire key, or add the key into the design
• You can use a different filament without having to pause the print at a certain height to switch it out
• It's fun

What I landed on was a 3-part design where you have the key, the pretty top part, and a connecting piece; if the pretty top part is flat on top, you can attach the connecting piece to the bottom of it directly.

I've included a few sample designs to go on top of the key, but it's easy to adjust a design to make it work! All you need to do is resize your design to the size you want, and then cut out a cylinder from the bottom with a 6mm diameter and 2.5mm height (1.5 for the smaller connector). Or, if your design is easier to print upside-down, you can instead attach a hexagonal prism to the bottom which inscribes into a 6mm diameter circle with a 2.5mm height.

Update 4 June 2019: I added another type of connector I made for a specific model. I printed out this skull model:
https://www.myminifactory.com/object/3d-print-free-detailed-skull-93540
and decided to make a connector that fit it without having to make changes to the skull itself; it fits into the foramen magnum in the base of the skull (I had to look that up; it's the big hole in the bottom).