Are there ways to improve the safety of my homemade waste oil furnace?

Yes, you can improve the safety of your waste oil furnace. First, strain the oil, to clean as much of the contaminant out as possible. Then, there are electrical systems (essentially oil-based batteries, that attract nasty stuff to the anode or cathode), that clean up oil. Then, you can work with a high-heat furnace, that delivers not only high heat, but also maximum airflow. And, finally, you can modify a catalytic converter system to work with your system, so that any exhaust that comes from your system is cleaned up. Just remember catalytic converters run hot, so they should have stone beds of their own, away from ignition sources.