I highly suggest soldering the flexwatt to the cord directly. Clips can come lose and/or not completely connect.
Personally, I remove a centimeter or two of the plastic off the flex metal strip then solder on the electrical cord.
The exposed wire on the electrical cord does not need to be as long as the metal clip. As long as a part of it is attached through the small loops of the clip and secured down(clamping it tight), it should carry the electric current fine.
I've had flexwatt soldered and clipped. The clipped flexwatt never got as hot as the soldered one and both were on the same thermostat. And I had some trouble with the clipped one as I had to scratch and puncture it a few times to get it secure. Even though it was connected, if the cord was pulling the clips a certain way, it stopped working. So that's why I like soldering it.