Hi Michael,
You ask some darn good questions!
First and foremost, you need to know some specifications about your speakers so you can build the boxes the right size. Each speaker model is different, and some like big boxes while others prefer small. If you build a box that is too small for the speaker you have, you will not get any low bass and it will be kind of boomy. If you build the box too big, you won't get enough of any bass, and the speaker will get easily damaged if you try to play it too loud. Those specifications are called the Theile Small parameters, and it's three numbers that the manufacturer can (hopefully) supply you. If not, perhaps you can calculate the size of the original box the speakers were made with?
The rest of the questions are less crucial. You don't need to use pressboard, but it makes a great enclosure material because it's dense and cheap. Dense is good because it won't resonate very easily - you want the speaker cone to make all the sound by moving air, not from a cabinet that vibrates or flexes. Pressboard is too heavy for DJs, so their speaker boxes are usually made of plywood - which is rigid but not as dense and if there's a large void between layers it can develop a nasty rattle (I've never had troubles with plywood in my car boxes). No matter what material you use, you can make up for lack of hardness or mass by damping the inside of the box and/or installing extra internal bracing to make the largest walls more rigid.
Finally, speaker shielding is only necessary if you want to place your speakers close to a tube television or CRT monitor. Without shielding, the speaker magnets will mess up the screen on cathode ray tubes. It's a pain to add shielding, but it can be done.
