Nice solution, never thought of using a zip tie like this. For plastic where the back is not easy accessible i always made the hole smaller by melting the edges with a soldering iron, and the cut off from zip ties as filling material. And if the back was accessible, i used one of those little U shape metal plates that have threads one side, and where the U normally is placed over a hole on the edge of a metal or plastic sheet to fasten it to something else. These are very common on bodywork on cars. The rivet nut that you mentioned is also a nice alternative for this problem. Another reason for not throwing away zip tie cut off's,they will become in handy soon or later.
