I've just purchased a new fan heater for $12. It may be overkill in terms of size or it may end up delivering a lukewarm result. Will wait and see. The nice thing is that it fits on top of the slow cooker (no wobbles so won't topple over).
Another benefit with this fan heater is that it potentially can be hacked without burning the house down. At the bottom of the heater there is a cut out switch which keeps power on if pressed in or switches power off if released. Just need to find the right relay to replace the tamper/kill switch.
Next part of research will be evaluating temperature profiles based on heat settings and fan speed.