In one of the classes I took we were trained on creating a field expedient dressing for treating pneumothorax: Cut a panel off a plastic bag and place it on the wound, taping it on three sides or on four (depending on the protocol for EMS where you live). Three sides allows the flutter valve to work; four sides keeps the problem from getting worse. Either is almost certainly better than nothing.

I do have a purpose-made chest seal in each of the car kits as well as my range bag kit.