youll have to excuse me because I am looking at this from my phone (can't see the model), but this appears to be a fairly simple case where multi body techniques would come in handy.
I'm making a few assumptions here, but if I knew what that pocket geometry was from the get go, I would use multi body techniques to construct a male "core" or "punch", as a simple solid, which I would later subtract from the model. Here's how:
1) model the core/punch first (as a solid), using the same techniques that you would model the outer body.
2) cut extrude the sides (the pocket basically appears to be a truncated version of the outside - from the photo at least), then I would
3) use the fillet tool to round the corners of the body (the 2 large fillets at the right and left of your pocket), then
4) again use the fillet tool to to the minor fillets.
5) Use the tree to hide the body
model up the outside As a separate body, careful not to combine with the first, if the first is hidden, I think that should be pretty easy, but again on my phone, so not sure at the moment.
when you are done with the outside body, show your core or punch body from the tree
use combine to subtract your pocket from the outside body.
Let it me know how that goes!
I am not sure what type of cavity you are after..
If you want to split into core, cavity and slides, try using mold tools within SolidWorks
..as Brandon has generally said... it's fairly straight forward if you plan what needs to be used to get your halves..
Lash Point - zxys.zip 7.8 MB