I use the plastic for sale signs, they are thin but work not to bad just have to be careful when pound hole in them. also i use poster board once draw out the pattern i use box tape to cover the pattern then cut it out.
Nice harness is there any chance that i can get a copy of this pattern aswell i have a american staffordshire terrier he is around 90 lbs I have been trying to make a harness custom fit for him