Where I live it rains on average of 155 days annually so good rain gear is a must and I always carry them in my pannier. I commute everyday on my bike and I also enjoy recreational cycling on weekends and holidays switching between the full size commuter and 20" folder. Mounting panniers on the commuter is easy with a rear rack but on the folder it's a different story. On folding bikes much of the rider's weight is on the rear wheel due to the smaller geometry and upright sitting position so I want to hang the pannier on the front to balance. However, I don't want to purchase another set of panniers because it would be much more convenient to use my current pannier (no need to transfer content), saves money, and front rack interferes with folding. Additional benefits are it acts as wind shield and you don't need to dismount from bike to access the stuff inside. So, I'll show how you too can make your own handlebar pannier mount for under US$10 (assuming you already have the drill and screw driver).
Things you'll need:
Aluminum rod 5/16 inch diameter
"L" shaped corner braces 1 inch x 2
Stainless steel clamps x 2
Drill with 5/16 inch bit
Flat head screw driver or monkey wrench
Rope or paracord 12 inches long
Step 1: Measure
Measure distance between hooks on pannier. Mine are 3.5 inches inner and 5.5 inches outer so I'll need to cut rod at lease 6 inches long.
Step 2: Drill
Enlarge one of the hole diameter on corner brace with drill to 5/16 inch to match thickness of rod.
Step 3: Assemble
Insert rod into braces then mount onto handlebar as shown but don't tighten clamps yet. Measure distance between braces to make sure they will fit evenly between pannier hooks. Lightly tighten clamps and test hang the pannier. Make adjustments as necessary then tighten clamps. You shouldn't have worry about the rod coming loose because if you drill the holes to match thickness of rod properly then the slight curvature of handlebar should force braces to angle which into turn will hold rod firmly in place.
Step 4: Secure
Cut piece of rope about 12 inches long. Tie small loop to catch bottom hook of pannier then tie rope around stem or head as shown making sure there is enough tension to keep pannier from swaying. I have used this setup on both my folding bikes and they work great and hope it'll work for you too.