Don't want to pay money to have a snazzy Camelbak? Leaving on a week long camping trip in less than a week and realized you really wanted a convient way to carry 100 oz of water? I take a bladder for a Camelbak, and make a backpack sleeve for it.
Step 1: Get a Bladder
I don't recommend DIY-ing this part, as most glues and plastics aren't food safe, and you will be filling this with water. I didn't have time to get one shipped to me from the internet, so I went to a local camping store and bought one there. I purchased a 100 oz bladder as the price difference as compared to the 70 oz is a few dollars, and you don't have to use the full capacity if you don't need it. Mine was $27.
Step 2: Get the Sleeve.
For the sleeve you will need some sort of large envelope. An enterprising person could likely find these in a dumpster somewhere. If you can't do that, they are pretty cheap. Two of them ran me about $6, and I only used one. This is what the bladder will be siting in most of the time, so expect it to get wet and beat up. With water resistance and durability in mind, I went for some plastic 3M bubble mailers. I figure that will add some insulation also, and wont be adversely affected by water. A second bonus is that you can use an iron to heat weld the seams, rather than having to find the right adhesive. I sized the envelopes by dragging the bladder into Long's with me; measuring it beforehand also works.
At this point, you can also gather the rest of the ingredients. You will need.
A Wooden Spoon (optional)
Industrial Strength Velcro
Other fastener of your choice (optional)
Step 3: Make a Pocket.
The envelope you are holding now is too big. with all the extra width the weight can shift around, and thats useless. In this step you want to get the envelope to fit the bladder like a glove.
Look along the seams on the envelope, one side will be perforated. I decided that I didn't like these, and cut off of this side. Measure the width of the bladder, give it another 3/4 th of an inch, and that is the inner width of the new envelope, "X". Mark a line distance "X" from one seam. measure another 3/8th or so out from this line, and make another line parallel to the first. you now have two lines running the length of the envelope. The first line demarcates the inner size of the envelope, and the second one is where you will cut. Cut it now.
Put the piece you cut off to the side, you will need it later.
Put a fold of waxed paper along the cut edge. This will keep the molten plastic from sticking to anything but itself. Run the iron (I set it to about halfway, at the "polyester" setting) along the edge, melting the plastic together. Do this on a heat resistant surface; you will want to press fairly firmly to get a good seal. you will know when you have a good seal, the bubbles will have all popped, and it will look very fused.
Ignore the top flap for the moment, that part comes in later.
Step 4: Cut the Hole.
So, this was the biggest pain in the rear of the whole project. The hole needs to he a nice fit, because its lower edge is what keeps the whole 100 oz of water and bladder from sliding to the bottom of your pack. Stick the bladder into the pouch, with the bottom edge of the bladder touching the bottom edge of the envelope. you will be able to feel where the right angle for the hose comes out of the bladder: Mark a line starting from a little below the elbow to about an inch above it. This is where you will make a cut so the hose and elbow can stick out. Now, keeping everything in the same position, mark where the fill cap is. To do this, I think the best way is to unscrew the cap, and pop of the black ring. this leaves the clear plastic ring that is attached directly to the bladder in place, and its easy to feel exactly where that is to mark it. mark the envelope along the entire circumference of the ring. The black ring sits on top of the clear ring, so you can use the marks you just made to position it and trace around it with a sharpie.
Pull the bladder out, put it back together and then cut a giant U out of the envelope, where the lower part of the U is composed of the bottom half of the sharpie circle you just drew. The vertical portions of the U should be tangential to the circle and normal to the top (open) edge of the envelope. You only need to cut this on the non flap side.
You can also cut the line you drew earlier for the hose.
Step 5: Extend the Flap.
The flap is critical, its a load-bearing structure.
Take the excess you cut off of the envelope from before, it should be pretty long and a few inches wide. Cut the seam off, and then chop a length out of it that is equivalent to the length of the flap on the envelope. Put this section by itself in a fold of waxed paper, you want it to be flat and entirely covered by the waxed paper. Heat up the iron again, and run it over the plastic until you have melted all the bubbles, and its just a thin flat sheet. Peel the adhesive strip on the envelope flap, and use it to attach the new piece of plastic. If you want you can use waxed paper and the iron again to melt the two together. Use the same trick from before to mark where you need to cut the half circle out of the flap. This lets you fill the bladder w/o having to take it out, and makes for a cleaner look in general.
Step 6: Velcro.
This is pretty straightforward. Get everything put together and in the envelope in the right places, make sure your cuts fit well and the bladder lies flat. Pull the flap snug, and mark where it ends up. Cut some Velcro off the roll, about 1"x2" works well. Stick it together and peel the backing off the hook side. Stick it to the envelope where it will be covered by the flap, and then peel the backing off the loop side. pull the flap into position, make sure its square, and then stick it to the Velcro. Doing it this way makes sure the pieces match up, and is easy. I like to put the hook side on parts that will not have a lot of contact, so that the softer loop pieces are the ones that are on the flap and will be hitting the bladder. On the back, add two pieces of Velcro, hook side on the envelope. a big piece right at the top, and a small one at the bottom. If you have extra, a bit in the middle couldn't hurt.
Step 7: Velcro Cont.
Grab whatever bag you are going to be mounting this in. I used a random backpack I have. Hypothetically, you could use any bag the system will fit in and still have the tube reach your mouth. Get the envelope lying flat where you want it in the bag, and use the same Velcro trick from before to apply the loop side to the bag.Keep in mind there will be several pounds of water riding in this, so make sure you put the Velcro on a stable location. I am still working out the optimum general solution to the backpack mount. Velcro is about the only option I had in this bag because there wasn't really room for anything else. But webbing and clips would be a far better way to do it, as the Velcro makes me worried that the load will pull off and become free floating if I jump off an epic ledge or experience some other high acceleration event. The low positioning of the top piece of Velcro gave the bag way too much play, as a result it had enough leverage to peel the Velcro apart. Putting a second piece higher up seemed to solve the problem, but a positive lock system (clips, D-rings and webbing, etc..) would be much better. Adding support to the back of the envelope would also help. A 2.0 version my come out soon, depending on how this version proves itself.
*EDIT* I loaded the pack this morning, and the bladder seems really stable, the stuff (change of clothes, 2 books, lunch) in the bag keeps it from pulling forward and putting leverage on the Velcro. Check back at the end of the day to see how it went. Update: I took it to work, and wandered around campus hoping off of stuff to see how it held up. The system worked flawlessly, the Velcro held and nothing exploded or fell apart.