As for the knee pads and thigh plate, because that's been slipping as well, I cement-glued some velcro that I got from Home Depot and plastered on while wearing the pants. Granted, this is not the safest thing to do, but the pants were SO thick that I assumed it was reasonable to do so.
Here is the completed project. Though there are some things to fix, some re-gluing here and some additions there. But I’m mostly glad that the “MK II” is done. ^^