Fiberglass or asbestos?

The vintage/age of the hotplate in question is important in determining what type of insulation you are dealing with...

Small fibers of either material are harmful to inhale... It is the small fiber particles that embed themselves into the tiny passageways in the lungs that causes severe health problems with prolonged exposure over time...

~~ Evan

Bought it about 3 years old.

BTW, I found the defect. Apparently one of those loose fiber got between the (mechanical) thermostat contacts. The solution is simple: I pull a piece of paper between the contacts and now it conducts.

I'm inclined to not put the fibrous insulation back in this thing to avoid more loose fibers. However, I need a 1-inch thick, heat resistant and slightly elastic material to push the heating sheet tight against the top surface.

I think several folded oven mittens may work but is there a better alternative?

That sure looks like fibreglass to me, why not replace it with some new fibreglass?

If it's 3 years old it's not asbestos. Nor does that look like asbestos. My guess would be fiberglass too, or some kind of rock wool.

Here's what asbestos looks like.

It is NOT asbestos. It is a form of fiberglass or mineral wool and can take high temperatures. It also serves a purpose so it should be put back. The wrong material or no material can cause overheating and damage.