So I have a big old BaileyWorks mess. bag that I love and have used forever, but my new routine involves hauling a laptop, some case files (I'm a lawyer), the usual complement of tools and camera and such, and lunch, and it was starting to make my back hurt. So I switched to a very sturdy, basic EMS backpack (2000 cub. in., one little pocket inside, lots of straps and buckles outside), which is, of course, not waterproof. In the long run, I'd like to invest in a waterproof bag (either the Timbuk2 Hacker or the Banjo Brothers commuter backpack), but until then, I came up with the following solution for a very inexpensive but sturdy waterproof bag liner. All you need is some kitchen garbage bags, some plastic shopping bags, an ironing board, an iron, and about half an hour:
(Instructions first, then pictures)
1) Find a size of garbage bag that is slightly larger than the bag you want to line. I used 13 gallon kitchen bags.
2) Place four of these bags inside one another (like double-bagging it at the grocery store).
3) Place the quadruple bag over the ironing board so that just one side is exposed.
4) With the iron at about 60% of maximum heat, press down on the quadruple bag. Don't drag the iron as you would on clothes, just press, lift, and move to another spot. The bags will melt together. You will probably make some holes all the way through. Don't worry about this.
5) When you have melted the four bags together on all sides, the plastic will feel tough and kind of stiff in most places. Find the spots where it still feels filmy like an ordinary plastic bag or where there are holes.
6) For each hole or weak spot, take a plastic shopping bag, fold it in half, and iron it on. Start by ironing the edges of the shopping bag to hold it in place, then do the middle. If little bits are left sticking up, just cut them off with scissors.
7) When your liner feels adequately sturdy, fold the corners or sides as needed to create the shape you want. Iron each fold into place - this may take a little more heat, because you're working with a thicker piece of plastic.
8) Reinforce any part of the liner by ironing on more bags. I did this on the bottom of mine.
9) Depending on how the liner will fit in the bag, you may want to make a flap (as I did). Just use the same technique to make a big flat piece of stiff, melted plastic, then use a few more shopping bags to create an iron-on hinge. Make the flap too big, then cut it down with scissors.
10) Voila! Insert liner in bag and ride in the rain.
Here's my masterpiece, outside the host bag, with flap open:
Here it is with flap closed:
Here it is in the backpack, with the flap open:
Ready to go: