not alot of hang on skimmers for a tank that big. does it have to be hang on? You could try precision marine, they have a hang on meant for bigger tanks. CPR bak pak might be good enough for a 120... it just depends. If you can get in sump everyone here reccomends Euro reef.
I use a Berlin Classic hang on skimmer .It works really well. I have never had any problem with it. Just need to clean it once and awhile but thats really pretty easy to do.And I think I spent about $225 for it with the rio pump.