WickedPdfTempfile.new use temp_path argument

You can use pdf_from_string with an option :temp_path. If temp_path available WickedPdfTempfile use this path or set :temp_path as default in WickedPdf configuration 

    WickedPdf.config = {
      :exe_path => '/usr/bin/wkhtmltopdf',       
      :temp_path => File.join(Rails.root, "tmp")
    }