@daox I couldn't post pictures so that's why. The project I saw was called meandherhr I believe, but I thought of something like this
Just a simpel Household radiator.
@ac-hacker i like the idea since it's simple to build, but I'm afraid a lot of dirt and grit like hairs will get stuck in between it. I also have a demand heater, but would setting the temp of the heater make more sense then reusing the heat. The heater is like 1m from the shower and has isolated lines.