MYOG roll top dry bag

If you cut around the circumference of the milk jug, you can get 19 inches, but it will be curved. If you fold that piece in half lengthwise, you can eliminate the weakness of the curve.

–B.G.–

For my roll top backpack I used a long strip of that heavy duty plastic ribbon like the kind that holds boxes of paper together at an office supply store or merchandise to a pallet. I got some that was about thumb-width from my local hardware store that they were throwing out after getting a shipment of stuff. Works great.

EDIT: to add picture

Yep, many foodstuffs come packed with that around 'em as well (apple boxes and other produce are notorious for this), so if you know someone in the food industry…

Be carefull when you decide to use the ribbon Greg mentions because they come in very different qualities. While there are some that are very good, most of these ribbons easily crack and fray lengthwise. This will reduce the stiffness to next to nothing and it can actually be pretty sharp. Sharp things and stuff that needs to be waterproof is usually a bad combination.

I've actually had some succes using just polyester webbing. Much easier to work with, no risk of sharp edges and plenty stiff (especially if you make a double layer).

I've used PET plastic from soda bottles, it's holding up really good to repeated bending. Cut it like a spiral and you can get it long enough. Heat it with an iron to make it flat. If you want it stiffer, fold it and heat again.

It's holding up really good to repeated bending

I'm liking Bob's thoughts about milk cartons, doubling (tripling) it over if you want more stiffness.

I often dumpster-dive at the recycling center, especially in the HDPE bin (milk jugs). Other products also come in HDPE but are thicker gauge – detergent, shampoos, coffee, etc. Cutting a strip from one of those would give you stiffer material.

For really thick, but cheap HDPE sheet, I always keep a few of these around:

http://www.amazon.com/Pelican-Snow-Flite-Roll-Plastic/dp/B002QZ2Q86/ref=sr_1_9?ie=UTF8&qid=1381425848&sr=8-9&keywords=snow+sled

They retail for $6-10 but are half off come Spring. Or $1 at a thrift store. For a quick, long strip; roll it tightly, tape the roll securely and use a chop saw to cut off a section of roll. It could 2", 1" or 1/2" thick. The cut edge will be rough, but can be smoothed easily with sand paper or those Stanley Surform rasp-like tools you pull towards you:

http://www.amazon.com/Stanley-21-115-4-Inch-Surform-Shaver/dp/B00002X1ZH/ref=sr_1_cc_1?s=aps&ie=UTF8&qid=1381426246&sr=1-1-catcorr&keywords=surform

Or mark along a straight edge with a sharpie and then cut your strip off with shop shears, or some heavy-duty scissors.

i met a gentleman at work who offered to make me a cuben dry sack to be used with the flash 45 i am modifying, i think i will cut up the next milk jug that empties and see how that goes

tap plastics will cut a 1/2″ strip of polycarbonate plastic sheet in 8′ length for like $2.75 cents. https://www.tapplastics.com/

You can buy three or four packs of plastic cutting boards that are really inexpensive and would do the trick.  I’v just been using 3/4 nylon webbing on both sides and it seems stiff enough for a roll top.

How about this idea to save materials – no stiffener at all!  On my bags, once I fold the top of the raw bag down two times to hide raw edges before sewing, it’s plenty stiff enough.  I then add snaps on the top and linelocs on the sides to hold the top closed and securely down.  Water doesn’t stand a chance of getting in.