I tried this for my two hatchies, I used smaller size coreflute as I had a spare piece laying around. I think I did the same as someone else did (I can't be bothered trying to find the post) and slit the coreflute along where I wanted the heat cord to go. To make it easier to handle, I used a strip of gaffa to tape it up again afterwards, so it didn't bend. Works a treat! Only problem is I've got about 2m of heat cord just flailing about the place now because it's way too long! (I only had a spare 25 watt cord, I could definitely go down to a 15 watt, just haven't got one yet.)