Super light and indestructible slop deflector field tested when i was a messenger in Montreal, waaay back.

Corplast, zapstraps, knife, measuring tape, coat hangers, pliers with cutter(not pictured) and a philips head screwdriver.

Roughly measure the distaance depending on how the fender is to sit in the frame. I like mine to use the holes in the frame but you can attach anywhere.

Make sure you know how much room you have in the frame to play with.
8 x 48

To be trimmed as we go
Mark the centerline and distance on either side to fit the frame.
Open the coat hangers and trim the curly bits off.
Insert the coat hangers to the left and right of your centreline with at lest an inch between them, for stability.
After trimming, to allow for the bend and the frame, rough fit to see if more cutting is needed.
Trimmed to fit, make minor adjustments, use the screwdriver to poke holes as guides for the zapstraps, strap it down and viola.

the fender can be bent to whatever angle you want. When you ride it might slap on the tyre if it's rough, but a tight bend near the frame connection can draw it up away from the tyre...BUT they won't fall and rest on the tyre like my seatpost mounted store bought. They aren't the prettiest things in the world but I've biked though many a year of nasty road crap and not gotten a thing on me..
Nothing will stick to them and they won;t collect gunk like some store boughts I've tried. They won't freeze and break in extreme cold either. The fender extends past the frame brace and curves forward slightly for clearing. I've cut through half the material on the drivechain side and bent it in, this will cause crap to not spray/collect near the chain when it's still frozen slush/snow weather, minimizing the amount of crud on the frame.
Custom fit to your bike and enjoy, cut off in the spring.