What I got is a slimline heater matrix which I'll get plumbed in and simply drop into the bottom of the tank. Not nearly as efficient as a calorifier, but it'll be enough to keep the frost away this winter.
My only concern is the matrix contaminating the water, so I'm going to scrum & rinse it thourougly with pure before it goes in, and put an extra DI after the tank just to be sure.