At $3.40 a foot its really hard to understand why you wouldnt remove your one foot section and install the proper three foot piece???
You might be able to hit the temps that you want BUT you are going to overwork your flex-a-watt and/or your thermostat. If you are lucky it will just quit working but it might make you a small camp fire too? Who's fault will that be??
The problem is not with the thermostat, X-amount of flex-a-watt is only going to heat x-amount of space. Kind of like trying to heat your house with a zippo lighter, yes it will get the job done but it is not enough to do the job. Now you open the front door and your house drops to freezing temps and takes forever to heat back up after you close the door. Get the point??

You can run more flex-a-watt than you think you need and your t-stat will make sure it doesnt go over set temp.