This is an easy way to make comfortable, durable, and low profile rail covers for accessory rails. Not only are they dirt cheap to make, but they're easy to cut to size. Note that they're one use. Once you take them off, they're pretty useless, but it's not hard to make more.
Step 1: Materials
You will need: Duct tape of your choice, HVAC tape (adhesive aluminum), Straightedge, Scissors, Razor knife.
Step 2: HVAC Tape
Measure the length of your rail segment and cut 4 strips of HVAC tape, allowing some room for error. You can always cut some off. This is doable with 3 strips, but I've found that 4 holds on better.
Step 3: Layering Tape
Peel off the paper backing on 3 of the sheets and carefully layer them on top of the 4th. This gives the rail covers some rigidity, while allowing them to bend around the rail and retain their shape. Be careful to avoid wrinkles, because this tape doesn't stretch. In the future I'm hoping to use a pattern of wrinkles to add texture and extra grip to these but so far that hasn't worked too well.
Step 4: Duct Tape
Now add a layer of duct tape on top of the HVAC tape. I chose a nice olive drab. Again, try to line this up as well as possible. The neater the better.
Step 5: Cutting
Cut about 13mm total from the width of the tape. You can cut either side to even things out, just don't cut too much. Your final width should be about 4cm give or take. These should fit standard rails but feel free to adjust if they don't work on a particular model.
Step 6: More Tape!
Add another layer of duct tape, and cut around the edges. Leave a little room on the outside. When you bend the edges of the cover around the rail, the extra room will keep too much of the shiny foil from showing through.
Step 7: Peel, Stick, and Fold
Peel the backing off of your cover, stick it to your rails on center, and fold the edges up. You can use your straightedge to press the edges into the rail to ensure a tight fit. In this case the cover was a bit wide. That depends on the height at which the rails sit from the body of your gun.