Step 1: Tools and materials
9 1/2" x 19" piece of Coroplast
or other corrugated plastic sheeting - most often used for making outdoor signage.
Some bungee cording (about 54", give or take).
My local Michaels store has 21 ft. bundle bungee cording on clearance, for about 74¢ plus tax per bundle.
A tool to poke some holes with
Using a 1/4" hollow punch and a hammer or mallet would be my recommendation to make nice, clean, round holes.
Optional: a #0 grommet setting kit would give your cords some added protection from wear.
or Xacto/craft knife, to cut other stuff with.
To print-out of this template (2 sheets)
*note: If you cut out and affix the shapes to the coroplast, there should be an approximately 3 1/2" gap in the middle between them.
A reusable (woven) plastic shopping bag or tarp
I thought this large plaid one with the zipper would work well for this, and they are easily found in either flea markets, or else those weird, quasi- flea market shops such as 99 Cent City.
double-sided weather strip tape
is the easiest method i've found for adhering woven plastic to itself. It forms a pretty solid bond, that is often stronger than whatever the first layer of tarp or plastic is.
Needle and thread
It is possible to hand-sew the entire bag together with a needle and thread, instead of using the weather-strip tape.
Otherwise, you'll find these useful to create a new zipper stop at the length of the bag, should the zipper of your bag need to be shortened.
Step 2: Make the bottle board
You can print out a PDF template for cutting / punching the Coroplast here.
Coroplast will cut easily with scissors, and the easiest way to punch holes is with a hollow punch tool.
You can use a drill bit, but it will probably tear up the plastic. If you do plan to use a drill bit, use a piece of scrap wood behind the plastic to get a cleaner hole. Grommets will cover ragged edges, but if you have the hollow punch tool they likely won't be necessary.
Lacing up the back panel with bungee cord is just like lacing your shoes.
I used two pieces of bungee, but you could just as easily use one.
I used some cord to tie stops where you see them to support the water bottle at the base.I never got my knot-tying merit badge, but it looks like a modified diagonal lashing, and so far, it's seemed to hold fine.
Hopefully you can tell from the photos how the bungee cords hold and support the bladder, but if you have specific questions, please ask.
Step 3: Outer cover
This is the basic construction I used to make the outside cover. My bag was larger than the reservoir, so I had a lot of leftover material and options for where to place tabs.
Step 4: The end result
The finished product came out pretty well, I thought. It is durable and holds water, which were my goals.
There is a difference between my end product and the drawing I posted, an extra piece of plastic covering the hose. I added that to disguise my botched job of cutting the hole for the hose too low the first time. In other words, I added it in the name of vanity.