Quantity   Component name
1 × Snorkel Face Mask £12
1 × Car pollen filter £4 makes 10 filter rectangles
2 × Tie wraps I didn't have long enough ones to hand so used 2x2 in series.
1 × (Optional) Duct tape It you want to install additional internal filters.
1 × (Optional) Superglue/cyanoacrylate
1 × (Optional) Sawn-off snorkel If you want a more sophisticated filter mounting
1 × (Optional) HEPA filter material. I found some 99.97% filtration @0.3um