Having a sulk in the shed and mulling over my next move I spotted the original sheath to my bastard knife and had a eureka moment.
Removed the belt loop as surplus to requirements:
I then removed the rivets, punched 2 holes through both sides and stitched across the top and reinforced the stitches down what will become the front of the mask:
Next I removed the excess leather from the top and made a slit to join the 2 holes:
To finish off I just needed a securing strap, so I fished an old piece of webbing out of the craft box and cut to size:
It's not pretty but it's secure and effective.