E-mail: firstname.lastname@example.org + my PGP key.
You can also use my ProtonMail address: email@example.com
If you want to invite me to something Google-hosted (such as a Google Calendar, Drive etc.), you can do so with this account: firstname.lastname@example.org
Please note, that I'm not a big fan of Google services (or any other corporate solutions, really). If you want to know how to operate your project without Google, just ask me about possible options, I'll be glad to help. Let's fight the surveillance capitalism together!
Sometimes I also hang out on IRC (mostly Freenode) under the nickname lexik.
If we know each other, you probably have my phone number. In that case, you can also reach me securely on Signal.
Please note, that phone eavesdropping is a common practice in Czech Republic. Please consider other, more private ways of contacting me, before dialing my number.