I would recommend: https://missionpinball.org/ (Disclaimer: I am one of the maintainers). We recently started a collections of CAD designs here: https://hardware.missionpinball.org/. Another good resource is: http://pinballmakers.com/.
I also started a Youtube channel with some videos about building custom pimballs: https://m.youtube.com/channel/UCPXS1guuUIyC49ysa-nRuzg. I also stream on twitch: http://twitch.tv/jabdoa.
Jan