I've been thinking about using a home-brew heating jacket for a gentle background heat, they're designed for long term use, 240v and I think are thermostatically controlled. Down side is they're expensive but I find filament bulbs fail quite soon when left on for 12hrs at a time.
I'm also considering the heat mats sold for reptile vivariums, cheaper than the home brew jacket, available in a range of sizes and powers, 240v mains powered and designed for continuous use, I've got one still working after 20 years. You can even add a thermostatic controller.
I've discovered that stacking green wood in my shed, covered but not end grain treated (schoolboy error) has been a disaster this summer, it was way too hot in there so I'll have to rethink that.
Always something new to learn.
Steve