Flexible Heaters

I can't help you with your project but someone already makes such a product. If you are making them for your own use it might be easier to buy them. Let me know if you are interested and I will find out the name of the company.

Check out "pipe trace heaters".
I believe 'Chromalox`- (U.S.A.) is a manufacturer.
Units are available in flexible plastic tape and mineral insulated, (M.I.) cable types.
I would be careful about using silicone rubber for
such a purpose unless you projected outputs are low indeed.

Watlow makes these in standard and custom configurations in a few different types of materials. The metal foil is sandwiched (bonded) between 2 pieces of silicone with teflon coated wire exiting the sandwich for power connection. Silicone adhesive is available to withstand +300 degrees C so you could probably make your own.