Just my opinion but I think if you put clams up there all you will see is the side of the shell. Clams are a little pricey and look better when viewed from above. A cap will look great but shade areas below and again you will be viewing the underside of the color. I can't think of a good choice so if it was my task I would experiment with acro or other sps frags like a birds nest. They can always be trimmed and the new frags can be placed in my tank
Kari