I would ware welding glasses if I could... even though reducing the amount of light to your eyes decreases ones depth of field, I
feel more "relaxed" maximizing the light reduction. so the
darkest lenses I've found are Vuarnet PX-5000 (not recomended
for driving, but I do)
http://www.ej-sunglasses.com/vuarnet/brandpage-tec.html
http://www.contact-lens-center.com/Vuarnet/Sport.htm