Specifications

Size: 3" by 3" square
Nail hole: 1 x 1/4" diamater drilled about 3/4" from a corner.  This will allow the marker to hang naturally in a diamond orientation, making it highly visible and resistant to being bent by snow buildup.
Backing material: auminum, approximately 1/16" thick
Top lamination material: 3M Diamond Grade Orange.  This material is retro-reflective like highway signs.  Night time visibility is actually better than in the day time - up to 100m with a typical headlamp.

Nails

Aluminum is the best material because it is corrosion resistant, won't poison trees, and is safe to cut with a chainsaw.  Commonly available galvanized steel nails are hazardous to chainsaw operators, and the zinc coating can harm the trees.  The most commonly available type of aluminum nails are used for roofing.  These nails come with a rubber gasket on the shaft below the head.  They are typically about 1 1/2" long with 3/8" heads and 1/8" diameter shafts.

Cost

Expect to pay about $1.25 per marker at $0.07 per nail.  This price does not include drilling the nail holes.

Suppliers

Astrographic Industries (Surrey BC) for trail markers
Inprotect Systems (Langley BC) for trail markers and custom signs
Century Signs (Squamish BC) for custom signs
Dunbar Lumber (Vancouver BC) for aluminum roofing nails