which heat tape would be best?

[CD CONSTRICTORS]

C-Serpents uses 4" flexwatt bottom heat on their 41qt adult racks. Holds my temps just fine with 82F ambient temps. Hot= 90F, cold= 83F.

IMHO, 11" would be overkill and heat half the tub not giving much of a gradient from hot to cold side.

http://www.cserpents.com/C_Serpents/V-70.html