The one in my project does this:

/**
 * This configures the gradle build so we can use non-standard build file names.
 * Additionally, this project can support sub-projects who's build file is written in Kotlin.
 *
 * @param project The project to configure.
 */
def configureGradleBuild(ProjectDescriptor project) {
    final String fileBaseName = project.name
    assert fileBaseName.charAt(0).isUpperCase(), "Project must begin with upper case letter"
    final String projectBuildFileBaseName = fileBaseName
    final File gradleBuild = new File(project.projectDir, "${projectBuildFileBaseName}.gradle")
    final File kotlinBuild = new File(project.projectDir, "${projectBuildFileBaseName}.gradle.kts")
    assert !(gradleBuild.exists() && kotlinBuild.exists()),
            "Project ${project.name} can not have both a ${gradleBuild.name} and a ${kotlinBuild.name} file. " +
                    "Rename one so that the other can serve as the base for the project's build"
    if (gradleBuild.exists()) {
        project.buildFileName = "$gradleBuild.name"
    } else if (kotlinBuild.exists()) {
        project.buildFileName = "$kotlinBuild.name"
    } else {
        assert false,
                "Project `${project.name}` must have a either a file " +
                        "containing ${gradleBuild.name} or ${kotlinBuild.name}"
    }

    // Any nested children projects also get configured.
    project.children.each { configureGradleBuild(it) }
}

configureGradleBuild rootProject