04-16-2010, 08:40 PM
It's not clear if there is a stud on the right side of the opening. If there is, you'll only need to fill the other three open sides. If not, you'll need to do four. I'd just cut the pieces to the right size so they fit snugly on the inside and flush with the sides of the opening (use drywall or maybe 1/2" ply). As the span is quite small I don't think you require reinforcement or screws of any kind. Just use Liquid Nails to glue them to the inside of the walls. You might need a couple of THIN nails to hold them in place while the LN sets. After it's set, remove the nails and paint the inside of the opening a dark color. Then run your track across and place your portals. Make sure that the roadbed and track are a few inches longer than the depth of the opening so they span the opening and are firmly set on either side of the opening. Then, you'll be good to go....
Gus (LC&P).