I agree that blunt force trauma is not a humane method of euthanasia.
More importantly, CO2 generated from dry ice is also unacceptable according to the 2007 American Veterinary Medical Association's Guidelines on Euthanasia because the inflow of CO2 can not be regulated precisely.
Compressed CO2 gas in cylinders is the ONLY recommended source of carbon dioxide because the inflow to the chamber can be regulated precisely.