Build it yourself
Trusts the source and your build tools. Costs the build itself, and on iOS signing with a developer account.
1. Build it
Read the source and follow Build in the README. On Android, make a release build (debug builds are very slow on a phone).
2. Install it
On Android, install with adb install or with "Install an APK only" on the installer page. On iOS, sign and install it from Xcode with your own developer account. Then turn on airplane mode.
It goes with the ways of using the app in any of the other setups. It is signed with a different key from the author's release, so it cannot update a release install.