Good work. Passes the key criteria - being less stiff than the fender so when you wheel the bike off a sidewalk to the road to start your ride, you don't break the fender with your lever of a flap on the curb.

I don't run my flaps quite so low so they don't hit quite as much road debris but are still good and long. Been using drafting and graphics film (thin mylar and the like) for the past 20 years, cutting two flaps and taping them together with packing tape around the edges. This makes for a flap that is stiff enough to take the curl of the fender yet easily crumple on that curb hit and pop right back. I remove the fender rivets, place the flap between the inside of the fender and the bracket for the stays and secure with pop rivets or screws and nuts.

For me, making the flaps thin and with minimal fastening thickness is important because toe clearance is a factor on many of my bikes. Bigger tires also drives this. Fix gear doesn't help! When that fender hit from my toe is coming, there is no way to stop it except stop and lie down on the pavement. (Usually, not very gracefully.) I tend to go simply "Screw you, fender." And do better next time at clean fastener heads and minimal clearance inside to the tire.

Fenders. The great English invention. (?) I don't know that they invented them but they sure knew how to make good ones that worked! 60 years ago. With real flaps that worked. And the Japanese who made so many good bikes with fender eyes. Many sized for 27" wheels so running 700c, large tires and fenders worked really well.

Edit: Cut down water bottles are the popular Portland flaps. I always wonder how many curb hits with a flap that stiff the fender can take. (That is going to depend a lot on the fender material. The rather flexy Planet Bike fenders should be able to take many hits. The stiffer and more brittle SKS not so much. And stiffness, for front fenders is a real benefit as it cuts down on vibration, something that can be driven by flaps with curl that cause wind eddies at speed. (Curled flaps also are narrower, potentially more aerodynamic and channel the water better. Don't blow back at speed. Fenders with 2 sets of stays are far better at preventing oscillation with curled flaps. A good solid bracket at the fork crown also helps. (Here's looking at you, Planet Bike. Poor mount, flexy plastic and one set of stays. And thank you, SKS.)