As I was inserting the clamps into their slots in the calcium silicate plate I noticed that the slots were a bit too narrow, and soon they broke away. So I cut them away completely:

The carbon tube resistors can be widened and shortened as intended, but they tend to go back to their original shape:

And they also don't stay flat on the plate:

A colleague then said "It's moments like these when you wish you could just staple them down". Brilliant! However brittle the calcium silicate may be, that makes it actually very easy to push in some staples (sorry for the crappy photo, but the idea comes across I think):

The top aluminium plate was placed in a mold

and covered with silicone:

I also added a glass silk insulated thermocouple for measuring the plate's temperature. The aluminium bar is just for fixing the wires in their place until the silicone has cured enough.

The next step (attaching the aluminium plate to the calcium silicate plate and the resistors) can be done when the silicone is hard enough, but still a bit sticky - otherwise it wouldn't be possible to add another layer of silicone to the existing one.