Do those crossbeams stay in place if the wall is accidentally tipped forward?
I mentioned that you need some trickery to make them 'stable,' and that I leave such trickery for you guys to figure out.
Possible solutions could be - for the 5 wide window, just place something on the inside of the wall - like a mirror or something. It is a big photo but if you look here
You can see how I placed a standing mirror pressed against the wall that holds the tile in place.
For the cross beam, it gets a bit more tricky. You could replace the 1x4 tile with two 2x2 tiles, connected by a 1x4 plate. The plate would be on the side of the wall and would keep the tiles from falling out. If you then put something against the plate, like a bed or a desk or something, it would keep the tile from 'falling in.'
But these are only two possibilities - I bet people could figure out a bunch more!