I'm making an oyster shaped cake this weekend and would like some tips on making the top shell, either out of gumpaste or chocolate, I can't find a mold for anything like this so I'm going to have to kind of wing it...any ideas? Thanks in advance!

No ideas but good luck. Post a picture when you are finished.

How big is the cake?

What are you using for the top shell form?

How are you going to present the top shell?

Did I get it wrong and it's all the bottom shell?

the bottom shell is going to be carved from an 11 by 15 cake, so maybe around 10 by 13 ish by the time I get the shape right. I'm leaning more towards white chocolate for the top shell so I can swirl the colors I want a little easier, I have no idea how to mold it though, I'm thinking maybe crumbled tin foil in an approximate shell shape, crumbled to give the chocolate texture? It will be held open with dowel rods. Any other ideas?

I made an open clam shell cake a while back (in my photos), and I used pastiage for the top since it is lighter than any other material you could use.
I used the top (cover for one of my dutch ovens dusted with cornstarch) to get the right curvature for the shell. When it was sort of dry but not completely, I removed it from the lid and used fabric all bunched up to place on the inside and help the outer side dry as well. This can take several days. After it was all dry I used fondant to cover it and give it some life like creases on the outer side.
Then I attached it to the cake just like any other open box cake.
I'm sure there must be an easier way but this is what worked for me.

Thanks for all the help!