Toner-transferred the copper trace mask on the copper clad board. It actually turned out decently for the size and relative complexity. The edges peeled off a bit, but that always happens for me; I will redraw those areas with Sharpie or masking tape. I also scraped off the lingering bits of paper with a knife. For future reference, I used the iron on max heat, let the board cool briefly before setting it in the water, and let it sit in the water for ~20 minutes before rubbing off most of the paper backing, and 20 more minutes before rubbing off any remaining paper residue.