After reading your article on patching a hold in a tree trunk - you mention covering with a metal flap - does this have to have a hinge to open or is it permanent. If so, how do you attach to the tree - nails??. Using a screening and plastering over it brings the question - what plaster is waterproof? Otherwise the rain and snow would cause disintegration. Plaster is normally used indoors. Would appreciate your clarification on this matter. Thank you D. Coull dcoull@sympatico.ca P.S. This tree is a Whitebeam about 35 years old and otherwise has thick green foliage.
If you determine covering a hole is the correct coarse of action, you would attach the screen or metal with smalls screws or nails.
Home Improvement stores should carry a water proof plaster, exterior grade plaster.
https://www.gardeningknowhow.com/ornamental/trees/tgen/patching-tree-hole.htm