I was building an ergo split keyboard and was also interested in this problem. After a bit of searching on the internet, I found this: https://github.com/vluka

I was building an ergo split keyboard and was also interested in this problem. After a bit of searching on the internet, I found this: https://github.com/vlukash/corne-trackpad https://github.com/vlukash/corne-trackpad The author used a BlackBerry trackpad. In his blogpost he showed that it can be mounted on top of a keycap. I believe that you can 3d print a special keycap integrate directly with the trackpad.