Until something cost effective and safer makes it to the market, most of us are limited to flexwatt or heating cable. Used properly with a thermostat, risk should be minimal. UTHs will fail, too. Nothing is 100% safe.

To the OP, don't cut the 11". 4" flexwatt will work just fine for those size tubs. I use it for all mine, from 12qts all the way up to 41qts with no problems at all.