1. no you don't have... to you can constuct the port in the middle it's just usually easier to usean existing wall.

2. I would stick with 3/4"

3. no calcs for this one man. for the slot port it would be L x W x H and don't forget to account for the .75" the wood displaces. For the bracing it's basically the same thing you'll just have to measure the size of the hole you cut out and subtract that from the amount the whole piece of wood displaces. So it would be L x H x W(.75") - the hole in the middle...