Well, this is way late (as in 3 years late!), but I came across this thread when I was looking for the same functionality. I don’t like using 1-size-fits-all type of apps such as CoRE because I feel they’re often too complicated to set-up and use too many resources to perform simple tasks. I may very well be wrong with this statement, but I use it as my excuse to challenge myself and see if I can write my own custom apps tailored specifically to my needs.
So that being said, when I was looking for a simple app that I could essentially use as a fancy timer for my car’s block heater and didn’t find one, I decided to write my own. I realize I’m 3 years late and you probably found a solution that works for you by now, but I figured I’d share it anyways in case anyone else comes looking here for a solution other than CoRE.
My Block Heater app is available on my GitHub. I recommend that you add my GitHub (astrowings/SmartThings) to your list of repos in the SmartThings IDE rather than just copying the code so that you may sync the app as I update it. Feel free to give it a go and let me know what you think. I’m very receptive to suggestions for improvements and would also consider requests to tweak it some of its functionality for specific needs.