I actually put a little grey belt-loop on the back of the knee of my flightsuit that I feed the elastic strap thru. The strap has thick-paperboard on each end, stapled and sewed (gotta clean it up, just kept adding to make it stronger). One end is glued into the knee armor and the other has one of the big snaps they sell at Walmart. Then I have a little pillow (fabric around padding) that has velcro to go inside at the knee to make it tighter and more comfortable.
My loops are kinda low, so the armor slides down a little below my knee when I'm standing. If I'd put the loops in the perfect spot and had the elastic the perfect length, this would be the ideal way to do knees. Although, letting it slide above or below your actual kneecap will keep the stress on the elastic lower. It tightens when you bend at the knee, so that snap is gonna want to pop off when you stoop to put your boots on.