Any "reasonable" methods would be appreciated. I have heard that this can be done but I'm dubious that it would do much. Maybe if I put polyethylene on the right bits?
I think the greatest efficiency improvement can be obtained by cleaning the condenser coils on the bottom and/back . In a home unit the coils,especially on the bottom collect a lot of lint , pet hair , etc. Vacuum them annually or more often. I have also added aluminum window screen to protect the coils from lint. If you use foam ,I think you will find polyurethane and polystyrene easier to obtain.