Actually those silicone heaters were made with pretty dense material used to cover the heat elements. there wasn't anything spongy about them.... But... i used aluminum pressure strips against the plastic wrapped glass.
I built my first couple 100 bows using them, before i went to the battery charger system with stainless steel strips. I haven't used a hot box system since 2007-2008.
I think there should be someway to use a flexible nichrome material for heat strips with a different type adjustable voltage regulator to regulate the temp. i'm only getting about 200 bows out of a battery charger before it pukes on me... Its quite safe using this system, but it's definitely battery charger abuse... But the stainless steel is very durable and you can sand the glue boogers off them when needed.
:Kirk