The 3 in. heat tape in a Cserpents rack is just there to make a hot spot, the other side of the tubs will be close to or at room temperature.

You could raise room temp a couple degrees to get the cool side of the tubs to where you want them to be.

Your idea to replace the 3 in. with 12 in. heat tape could work, but I'd be worried about the tubs rubbing through the heat tape and causing a fire. Also I wouldn't cover half the tubs depth with it, no more than 1/3 the total tub length should be at hot spot temp, there needs to be 2/3 unheated/heated less to allow for a gradient of some type.

Does your rack have the open sides or enclosed sides? If it's open sided, just insulating the sides could get your cool side up to temp as well.