This is what I ended up with:
I'm pretty sure you could get by on a lesser voltage converter but I wanted to get one that wouldn't burn out (ie has significant overage capacity and a thermal protector) and was, at least in theory, safe (ie UL or CE listed). I got this one on Amazon.
The heater is truly nano (precisely 4" long):
Shown versus the Catalina 50W titanium heater and the Visitherm 25W (you can tell I've wasted alot of money trying to find the perfect heater for this application):