Silicone loses its stickiness after a while because it picks up…. everything.
If you can’t sleep still, then you can try sewing pad loops onto your bag like what Kelty does. They have ‘Sleeping pad security loops’.
From there you can tie your bag to your pad. You can even sew Velcro strips to the bottom of the bag that are long enough to reach around the pad and hook to each other.

Sometimes I use my Western Mountaineering POD 30. It doesn’t have down on the bottom. It has pad attachments built in. This saves weight.

But once you attach your bag to the pad, you can’t readily sit up in it. But this has not bothered me.

-Barry