So the biggest reason you don't want to stick with the pad is the cold?
To keep the crate warm, I would:
Pack the area behind the separator with blankets. Open air will create a draft, whereas blankets will help trap his body heat and radiate some of that back to him.
Put the crate itself on on something soft/warm. Try to minimise the amount of heat lost through the bottom of the crate.
Keep the crate away from windows or doors - anything with a draft.
If it's still cold inside the crate, you could try lining the inside with something like this. Make sure not to cut off airflow; but again, the idea is to reflect his body heat back to him.
If you are concerned about comfort, you could look at getting a firmer bed with a waterproof cover like this, so that he can't bunch it up but it still provides some warmth/softness.