There have been a number of enquiries about fitting the strap on the half masks without poking holes in the mask itself, so I thought it might be helpful to detail this alternate strap fitting process in five easy steps. All you'll need is some excess card from your build, scissors, a ruler, a skewer (or a sharp pencil will do) and a length of elastic long enough to fit your head.
1) Cut two small squares (approximately 35mm each) of card
2) Poke two small holes in the cards with a pencil or skewer
3) Thread elastic string through the first hole from the front of the card, then feed the elastic through the second hole from the back to the front and secure the loose end to the rest of the elastic by gluing, stitching or tying as shown
4) Make sure you have the right length of elastic to provide a snug fit before tying off the second end
5) Glue the card pieces to the inside of the mask