I had a through wall a/c for the past few years in my apartment. When it broke down this summer I removed it and ended up buying a window unit instead as it was cheaper and I am in a rental. However now winter is here and the plastic bag I covered the hole with is insufficient and letting in cold air.. how do I cover it properly and insulate the hole... https://www.askmehelpdesk.com/images/smilies/confused.gif

You can stuff insulation material around the unit. Then bag and tape off. If you can get to the outside of the unit the same can be done there if necessary.