IMO, the tank is too bare and that may be the way the clowns feel too. HOWEVER, the O2 situation should be the first thing you try to solve before decor. That just might be the problem here and the rest is moot.
As for possible other decorations, there are synthetic anemones that you can buy which may help their psychy (if you can't do live anemones) as well as types of artificial plants you can put into the tank to make the bottom more appealing. As for the caves, they are not really "cave" dwellers so you would be better served to put a back to the cave so that nothing can get them from behind. Just a little indentation for them to get in to may be all they are looking for.
Here's a pic of one of my old tanks. It's a 75 gal ( which I think is a bit bigger than your tank?) but you should get the concept. The thing to know is that by decorating this tank the way I did, many of the fish that are known "scaredy cats" and like to hide, are visable all day long because they have the security to go to if they get scared. This pic was taken just about 10 minutes before "light's out" so many of the fish were already in bed but there are actually over 20 fish in the tank including Angels, Butterflies, Tangs, Gobies, Blennies, Sweetlips and Clownfish. 10 minutes later, not 1 fish was visable.
Aquarium Advice - Aquarium Forum Community - Andy Sager's Album: Clownfish - Picture
The area where the clowns are ( which was being blocked by the Majestic Angel
) is actually a piece that had some fire coral in the back and 2 types of mounding corals on the sides and a hollowed out center making it look like a baseball mitt. The clowns would sleep in there while the other fish went behind the decorations. Keep in mind, because of all the decor, I did need to put 2 power heads in the tank to make sure I had adequit circulation through the decorations.
Hope this helps