I took some 25mm straps and cut them to fit, The loops on the bottom require you to fold over the 25mm strap but I figured most people would just use a thinner elastic strap anyway. The top mount will support a 25mm strap though. I personally printed some buckles and used CA glue to secure when making the harness. I probably over engineered the harness but it works. I also recommend using a thicker elastic to go under the dogs jaw to help hold it in place. I am using some elastic string right now but plan on changing that because the string is uncomfortable for my pup and causes him try and get it off. making the mask more comfortable for the dogs helps with them wearing it. 

Everything else is the same. I printed with PLA+, printed upside down, used tree supports, etc. I changed the measurements for my dog but uploaded the original size file.