Tightly wrap 2" of high-quality electrical tape around the plug stem. Squeeze this as you insert the plugs and the tape should expand behind the bumper cover to hold them firmly in place.
Insert shrink tubing (insulation used on coax, phone charging cables) to provide a thicker sleeve around the stem, then install without issue. The tubing size will depend on hole diameter. Available at hobby shops and hardware stores.
Please contact us and we will see what solutions are available.