I use a neoprene diver’s hood to cover my neck while wearing the gas mask. The downside is that it’s yet another layer to wear so things get pretty sweaty!
With regards to the materials, this is going to depend on your prop making skill level and the range of tools you have, how much time and money you have or are willing to spend on the construction, as well as factoring how much potential crush-and-knock the props’ll get. We don’t travel anywhere interstate so our props aren’t deconstructable but they are pretty sturdy. And with sturdiness does come weight!
Looking at the Backscratcher and the Detonator, these are my prop-making partner’s thoughts: