I have a very bad back (2 herniated discs and one bulging disc). My doctor and PT are against heat on a back bad. Instead, they insist on cold only.
I have those flat, rectangular cooler packs for lunches. I strap those to my back with an adjustable back-brace I inherited. Works well and is infinitely cheaper in the long run.
Also, the Thera Cold pad looks to be $19.99 on WalMart.com.
Complication makes it easy to explain failure.