Haven't done mine yet, but was planning on snaps fastened to scrap ABS glued on the knee with mating snaps on the elastic. Then a sewn on piece of Velcro in the middle to help it stay in place.
I used a sew on style snap that was epoxied (male) to the inside of the knee armor & sewn onto the flight suit (female). I just put a hole in the elastic strap so that it went over the male portion of the snap. This has worked well & I just attach everything before putting the suit on. The elastic band can be made so there is some tension on it to make it look like that is what is holding the knee armor on.