Which caps or hats are designed to be worn...
Which caps or hats are designed to be worn under a helmet?
These might work, but this is what I suggest:
The wind stopper in the front is ideal!
the Castelli Risvolto Winter Cap is great if you want the wind stopper and warmth for your ears etc and you also enjoy wearing a cap year round. I have used skull caps under my helmet and they tend to rise up by my ears and I lose the protection and warmth
This hat fits snug and is the perfect winter hat