1) It looks like there is a rectangular cut out for a port on the back of the speaker. Does this area just stay open? I've always seen tubes connected to ports.
Think of the long transmission line that zig zag's through the enclosure as a really big port, because in a way, it is. It gets smaller is it reaches the end and the big opening in the back really is a big opening in the back. Most paint all the insides Black and leave it as is. I few people have put a grill cloth covering over it. It is DIY, you can do it any way you want.
2) The counter sync depth for the tweeter is .18 and the speaker post is .145 which doesn't equate to a standard measurement. (e.g. 3/16=.1875) This is true for the tweeter cut outs as well. (3.2 for the tweeter and 4.1 when including the counter sync section) How close should my measurements be? A little deep or a little shallow?
These measurements are made as per a dial caliper.
When cutting circles with a router and a circle cutting jig, you can easily set your depth to any level needed.
When making the 3.2" diameter through hole one can be +/- quite a bit and it still work out fine. So long as it is big enough for the tweeter to fit down through and leaves enough area for the tweeter fame to fit on.
The 4.1" hole for the tweeter frame needs to be close. Most circle jigs, like a Jasper Jig, allows for a 4.125" diameter hole to be cut, and this works out just fine.
You can always cut a sample into a piece of scrap to make sure it all fits fine. I often do.
