siy - Thank you very much for those links and tips.
I'll pass on a link to your post to my friend, and we'll see if we can try some experiments.
I've never seen the Density Toner before.
We have been using 'proper' laser printable masking film, which is translucent rather than transparent, and with the 'hatched polgon' approach has given extremely even dense black. It is about $1/sheet, so we haven't done as many experiments as we might like :-)