You could use 'foil tape', I use it to tape down the Flexwatt. Why not lay it down on the surface you put your roach bin on and direct the heat 'up' rather that from the side?
Regardless, the tape does not radiate heat very far at all, it primarily heats whatever surface it is attached to.
Any idea what the temp at the bottom of the tub is? I use a thermostat with the flexwatt. At 110 degrers, the inside bottom of the tub is about 95 degrees. The Dubias love it. That being said, I am in a very humid climate so likely I dont have an issue with drying out.
Do you have a temp gun to confirm the temps? An electric blanket may radiate heat more than the flexwatt?