Aluminum tape works very well. It is heat resistant so it doesn't loose its stickyness.
Your substrate shouldn't be very thick. The temperature difference between the top of the substrate and right on the glass should only be a few degrees. You want to measure the hottest point the snake can get too, right where the UTH is on the glass. If you measure 95* at this point, the temperature on the substrate should be only a few degrees lower.