I cut a hole in the wall... now what?

[TrainNut]

Allrighty then! I cut the first of two holes in the wall to run my double mainline through. Obviously I need to do something so that if I get a derailment while passing through, I don't lose the trains in the abyss below. As I'm not a carpenter and I know that quite a few of you are, I ask - what's the best way to finish this off? I'm not concerned how the wall looks from the outside or inside as both will have portals to cover. What I'm at a loss for is what to do in between the two pieces of sheetrock?