Overhanging wall pieces

I’m trying to build a traditional hearth and chimney structure, and for the most part, it’s good, but there’s one piece that won’t fit because it doesn’t snap to the bottom of a ramp to overhang like an exhaust hood. This is my first post, so I can’t upload the picture to make my suggestion more clear. So here’s a rough representation.
/| This
/ | is supposed
/ | to be a
/___| right triangle

There is a ramp piece on the hypotenuse of this triangle and a wall piece on the right. There is nothing on the bottom. The right angle piece exists, but it doesn’t snap in that corner without something beneath it.

By right angle piece do you mean the sloping wall? Are you saying you cannot place a sloping wall piece without something beneath it first? Have you tried placing a ceiling tile as the bottom the hood first? That should attach to the back wall piece. I think you can then place the sloping wall on the ceiling tile.

Yes. I mean the sloping wall. I’ll try your suggestion.