I've been using knee pads since I was about 14 (now I'm 44) years old working for a carpet laying company (neighbor). Find something that is comfortable on your knee caps. glue or velcro extra padding if you feel it is necessary, I usually put in a couple of rags under the knee side of the pad.
Here's the most important part, don't use the top straps to secure the pads to your knees. The lower straps above or near the tops of the calves will keep them in place just fine. Using the top straps will just bind up and cut off circulation to the bottom of your leg, making your job more of a pain than it already is. Good luck with those knees.