Now I'm not sure this is going to work as I've only just got this all set up but here we go. I bought one of these

It's a 11"x17" terrarium heater with a fearsome 25W output. Luckily this has one good point....it's self adhesive so sticks nicely onto a conical cone...handy that.

Performance-wise it seems to be able to raise 30L by 1C an hour. Which is hardly blistering but it IS A 25W heater. The idea is to keep a beer warm enough in winter to ferment - hopefully it will work.
I'll give some more feedback when I've done some more tests.