Yea white probably isnt the best choice, but its easy to get a hold of matboard. There is also black, but thats a tricky one. The ones with black on one side and white on the other are nice too.
Flat yellow like they have on some easels, and speed easels would work nicely as well. But honestly when burning with a stopped down lens to try and get maximum working time, the amount of reflected light is so low.