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

[nachoman]

What size is the hole? It needs to be sealed up, trust me. Not just for trains, but for insects. My parents used to have a bad cockroach problem. When they remodeled their kitchen, they found where a careless electrician made a huge hole in the sheet-rock behind the wall mounted microwave. the roaches were using it as a portal to the ceiling, and through the ceiling vents to the outside. Since they sealed it up, there haven't been any more roaches in the house.

Depending on the size, I would suggest a few things.
1) You could use a piece of square PVC rain gutter downspout. Shame to have to buy a whole 10' length, but maybe you have one laying around.
2) You could cut the hole round and use a large PVC pipe coupling. Looking at your photo, the wall stud may interfere with that.
3) If you walk down the aisles at home depot where they keep the vent pipes and such, perhaps you can spot something square you can use as an insert. Perhaps a plastic electrical wall box?
4) If you can't find anything to insert inside, you could frame off the inside yourself with 1x4 wood. Cut the bottom 3/4 of an inch longer than the width of your hole. Drill a drywall screw partially into the side to use as a handle to hold it in place while you fasten it inside the wall. Then, put it into the wall, hold it in place using the drywall screw handle, and screw it in place using drywall screws. Do the same for the other 2 sides (the 4th side is the stud), cut off the drywall screw "handles" with a hacksaw, and tape and spackle the edges.