Personally I'd just make them plates, with the individual panels glued to the plates themselves. Then go in after the fact and add the lacing for decoration rather than function. My material is "pvc foam board" which is found under "Komatex" at www.sdplastics.com
As for the knees it looks like it is actually part of the shin armor and should not need restraints put onto it, however if you want that flexibility you can always put heavy duty elastic on the back of your knee joint. It's worked passably enough for me in the past.
I detail a lot of the plastic armor making on my blog.