To sew the pieces together, I used the Speedy Stitcher sewing awl, which, while faster than a normal sewing needle, did not have the speediness implied the name. After sewing the pieces together, I intended to shape it by wetting the leather then placing it on a form. Attempts to do so failed, so I shaped it by adding darts in the mask, creating a bowl-like shape that sort of fits my face.
The filter is a grate that I found at the hardware store, attached to a sleeve of leather with nuts and bolts. I attached the filter to the mask with brass L-brackets, which ended up looking quite nice.
The lenses are plastic circles from old welding goggles glued into metal gaskets that I found at the hardware store. The gaskets are attached to the mask through three drilled holes.
The strap fastens using two D-rings I bought when I bought the leather. The strap is just a strip of leather cut to the right size that folds back into the D-rings.
Overall, I spent about $30 and 40 hours on this project.