I went to my local hardware superstore and located the section with wall insulation. I Acquired a sheet of 2" thick wall insulation board. I also acquired some pipefoam, 3/4" being certainly just fine for shields. 1/2" will work in this case too. 4 long bolts and nuts, 8 very large washers, and a cabinet door handle that I liked the feel of. Throw in an old suede belt from a thrift store and tape, and you're ready to start.
I then did some math, and a little guessing. You want to build your shield so that the total size is under or meets the maximum. Measure the diameter of your pipefoam and divide that by two. Whatever number you come up with, you'll want to cut the foam board that much under what you figure for your end dimensions. (Generally, this diameter should be, surprise surprise, 2". This means your pipefoam will end up sitting flush with the front and back sides of your shield rather than having overhang that will pull up the tape.)
Cut the pipefoam in half. Using small strips of tape, secure it around the outside of your cut foam board. Figure out where you want your grip and arm strap to go. Using a screwdriver, carefully punch/drill holes for your bolts to go through. Put the head side of the bolt with one of those nice wide washers on the front sides. Secure it on the back through the appropriate piece with another nice wide washer and the nut. This is the tricky part. You want to tighten them securely, but not break anything. Generally, once the washers *just* start to bite into the foam board, you're at about the right point. You may have excess bolt on the backside. You can cut that off with a pair of bolt cutters or leave it. I haven't had any issues with leaving it, but some people squick just looking at it without handling it first and realizing I'd have to be trying to hurt myself to do so.
As an alternate grip and strap method, you can cut bar-shaped holes through the shield and just use a leather strips, sewing it into a continuous loop. I have shields constructed both ways. You can also go for a center grip. The shield will be light enough not to tire you out quickly in alliance-speed combat.
Once you've got your grip and strap in place, tape it up. Front and back. If you're going to put any heraldry on the front, now's the time to lay out the base. Then, decorate. Again, alternately, you can build a cloth cover to go over the shield and, again, I have shields designed both ways.
I'll see if I can't get a (bad) picture of my shield with heraldry tomorrow and upload it.
This is only one construction method and type. There are many more, some will make shields equally as light, some will make them heavier. There are many different materials you can use to make shields and this is only one set.
The materials here shouldn't cost you more than $30, and you end up with enough extra foam board and pipefoam to make different types of shields and/or have room to goof up on your first go at it.