I'm pretty sure you'll want to wait until after the glassing has set up to fit the piece, and from there figure out the best spots to mount the webbing and/or Velcro at. Cyano-Acrylate (Super Glue) adhesives should be more than adequate to affix them to the glassing, and if your velcro hooks come with a adhesive pad on them reinforce it with the CA adhesive.