Heating method - defrost element + bulb?

The fridge-freezer I am converting has a 20 W defrost element and a 15 W light bulb in the fridge compartment.

Can I use these as my heating method or am I better to get a tubular heater or similar?

Cheers,
Ben

Probably, but it is on the light side.
I usually recommend around 60W.

Also check that it is safe to run your defrost element for long periods of time.