I've built a lot of cabs over the years - probably about 30 or so. Here's what I learned:
1 - screws and glue with butt joints work just fine for smaller cabs, but for a 4x12 you probably want to at least use biscuits. Get a nice router for the edges, and use 3/4" multi-ply plywood.
2 - Although I'm fortunate to live in the town where Thiele and Small did their ground breaking research, I'd recommend that you don't use it. The cab will end up being much larger than you want/need.
3 - yea, porting is a black art - I did about 10 porting experiments. The best of these sounded fine until you put a microphone in front of them - then there were so many strange phasing effects it made the cab unusable. I gave up.
4 - For cab dimensions, I used the golden ratio (1.618) for my cabs. This means that if the smallest size (the depth) was X, the next largest size (the height) was X times 1.618. Then the next largest size (the width) was 1.618 times the height. The theory behind this was it would cancel out a lot of the internal reflections in the cab which would get rid of any wolf tones and odd resonances. In practice this made some very nice sounding 1x12, 2x10 and 2x12 closed back cabs. Didn't seem to add much to open backed cabs.
5 - tolex glue is a pain in the ass to deal with. Use the water soluble brush on stuff available from antique electronic supply; don't bother with the spray on stuff from 3M. Use a sharp knife, and do your cuts after placing the tolex.
6 - Use canvas pliers to put on your grill cloth and use a lot of staples. Paint your baffle board flat black before you stretch on the cloth - learned this one the hard way :toothy11:
7 - celestion speakers are fine, but Eminence speakers are just as good IMHO and a lot cheaper. The Eminence GB12 is a very nice "greenback" style speaker and sounds better to my ears than a "real" greenback.
7 - T-nuts are your friend. Don't forget to put them on before your grill cloth.