You had a number of good suggestions.
- Get a thin panel or two of foil faced isocyanurate insulation (1/2") or so to lay on, foil side up. It reflects body heat and adds a little cushioning .....
- Never kneel directly on concrete. Always use some kind of cushion, even if it's just some cheap HF moving blankets.
I use a big piece of cardboard to lie on. It provides a bit of padding, a bit of insulation and it's really cheap. I sometimes get oil on it so I have to get a new one periodically.
For jobs where I'm likely to be on my knees I start off wearing a pair of cheap foam knee pads. I really hate kneeling on concrete.