I've got exactly the same problem: I'm building a fermentation cabinet for home brewing but the only way I've found to do it involves wiring a temperature controller to a 240v relay setup. I've done it, but I'm too scared to plug it in - I'm not an electrician so I'd be ignoring all these warnings telling me I need to know what I'm doing. I'd be really happy to buy a premade switching unit I can control directly via a pi.