Without drilling holes and somehow pinning them into the boots, you could try something stronger like GOOP! glue. It stays somewhat flexible when it's dry, but is very strong. If you still have a piece of the leather I would test it and stick one of the spikes on and flex it around to see what, if anything would tear it off. Just make sure to rough up the attachment point on the leather and the bottom of the spike with sand paper to give the glue something to hold onto. You can get GOOP! at Home Depot or Lowes. I used it years ago to attach the blades to my Gaffi stick and it's held those on.