You're welcome!

Right, inside dims.

0.06" = ~1/16", but the (~) means more or less, so if you're not a perfectionist, then round it off.

'Cook's choice'. I'm sure there's some math to calc an ideal distance, but I'm no structural engineer. I forgot to add that as long as the brace spans at least ~70.7 % of the distance it works for me.

Horizontal 'rings'?

The top/bottom front to rear braces are above/below the driver. What you're trying to accomplish is to provide the driver with a rigid 'platform' to allow it to work as mechanically efficiently as practical. The 'tie' braces between the vertical bracing can be more of the plywood scrap strips. For these to work their best they should intersect, creating an 'X' brace.

A T/S max flat alignment means that the system's half space F3 is as low as it can go. Since the horn is much more efficient than the woofer, it must be shelved (attenuated) to sound tonally balanced. When placed near a wall or corner it can sound too 'boomy' to some due to the extra acoustic gain, while others prefer it since it means less HF shelving. I just gave you a vent size and length range of adjustment that I think is flexible enough for you to find one that works for ya'll.

GM