Removes base from Fluxus theme portfolio URL structure to: /project-type/project-slug

user.php

<?php

function fluxus_custom_permastruct() {
  $project_types = get_terms( array(
    "taxonomy" => "fluxus-project-type",
    "fields" => "slugs",
    "get" => "all"
  ) );
  $project_type_matcher = join( "|",  array_map( "preg_quote", $project_types ) );
  add_rewrite_tag( "%fluxus-project-type%", "(${project_type_matcher})", "fluxus-project-type=" );

  remove_permastruct( "fluxus-project-type" );
  add_permastruct( "fluxus-project-type", "%fluxus-project-type%", false );

  remove_permastruct( "fluxus_portfolio" );
  add_permastruct( "fluxus_portfolio", "%fluxus-project-type%/%fluxus_portfolio%", false );

  remove_permastruct( "fluxus-project-type-default" );
  remove_permastruct( "fluxus_portfolio-default" );
}
add_action( "init", "fluxus_custom_permastruct" );

/**
 * Never under-estimate the complexity of Wordpress...
 * When using add_permastruct additional rules are created.
 * Those rules break URL matching. Since they are not used we simply remove them here.
*/
function fluxus_remove_attachment_rules( $rules ) {
  foreach ( $rules as $rule => $redirect ) {
    if ( false !== strpos( $redirect, "attachment=" ) ) {
      unset( $rules[$rule] );
    }
  }
  return $rules;
}
add_filter( "fluxus_portfolio_rewrite_rules", "fluxus_remove_attachment_rules", 10, 1 );