What makes a
good diffuser? Any ideas about how one makes one's own diffuser? Is
there an easy way to diffuse the light?

Tranmission efficiency. Diffusion angle. Cost. Color neutrality.
Strength and durability.

"Sign white" Plexiglas is a good trade-off of cost vs efficiency.

For the ultimate performance, use holographic light shaping diffusers:

http://www.poc.com/

Expensive, but offset by efficiency savings if properly designed.