At an outdoor show a few years ago, a vendor was selling machine threaded bolts for installing in pre-drilled ANGLED holes. He demonstrated how well they worked to nayone who would listen. It was compelling, but I never tried them. I DO however used 1/2" lag bolts screwed into some trees ON MY OWN PROPERTY. After the season, they' re impossible to remove. They' ve been there for years with no problems whatsoever.