Small bowl heater?

greetings,

I've a male betta in a 4 litre / (1 gallon?) bowl.
Like myself, he is not coping too well with the sudden onset of Autumn and
the drop in temperature, and has become lethargic.

He gets a full water change every week, the water is conditioned with ager
and left to stand overnight (at least) prior to the change.
I tried doing 50% changes every five days but only did this once as this is
when I noticed fin rot. He ended up losing a large chunk of one of his side
fins but with the help of Melafix, this is regrowing rapidly.

During the warm summer months, he was happy and healthy and was making
bubble nests all the time and was really active and responsive.

While he was getting over his fin rot, he was a little less active but
eating so much, he became 'pig-fish' but his fin was regrowing at such a
rate that I kept it up figuring he needed the energy for the regrowth.

Anyway, now he is not eating as much and has become much less active and I
figure it is because of the cold.

I've searched locally (Sydney) and as many AU websites but the smallest
heater I can find is 25watt, which I am sure is too high. I'm in a small
unit and am not really able to move him to a bigger bowl at this stage.

I'm toying with the idea of moving his tank to a side table near a 25w lamp
but I'm unsure if the light would be unpleasant/distressing to him.

Any suggestions as to the lamp idea or any small heater recommendations?

t.i.a,

- claire