The very often posted Curtiss factory photo of stencil spraying
shows ply (or light sheet metal) stencils with stiffeners.
Rubber was such a more strategic material than paper (or ply or canvas) that its use has long caused me to doubt this theory. Latex. Tap it somewhere in South America (or where-ever, the Japanese held Malaya and Borneo) dry it into sheets, pack and ship across hostile oceans. Then turn it into a disposable frisket? Hmm.. Not when there were hoses, gaskets, tyres and condoms and a multitude of other items to make.
Not to say it didn't happen, but I'm sure a search of rubber usage in WWII Britain would come to a dead-end when using 'masking mat(te)s' as a search criterion.
G