This is what I have used for the past 20 years
An army surplus wool blanket. Folded into quarters, you can kneel on it all day. When you have to work on a larger area, 1/2 fold will do. And if you want to protect a floor/carpet, open it up fully.
They run about $25- $30 at surplus stores.
Tried knee pads, bu they were always slipping around on me.
The only downside is the occasional joke about taking a nap on the job. Just tell them, "If I had used this 30 years ago, I wouldn't need it today".
Experience is what you have an hour after you need it.