Once you know what size of battery pack you are going to have then you can decide on what stock material you will need to build the canister itself. My canister is made from cast acrylic tubing with an inner diameter of 3 inches(76mm) and a wall thickness of 1/4 inch (6mm). I had dificulty sourcing suitable acrylic in the Uk where suppliers would supply small lengths, I have a local supplier (in Belfast) who wont supply any less than 5M lengths at about £60. In the end I ordered from McMaster Carr in the US who have a very speedy service even to the UK, however you will pay at least $20 shipping even on small orders. They supply a variety of acrylic (as well as absolutely everything you could ever want for DIY scuba ventures!!)in different wall thicknesses diameters etc. and best of all they will sell most by the foot. My canister measures 10" (254mm) so this worked out quite nicely. However with enough people interested buying locally in bulk is a good option.
you have two real options for the top and bottom. They both require a spigot, the inset portion. On the top this locates the crush face 'O' ring and on the bottom it gives extra strength from sideways blows. They can either be turned on a lathe or you can simply glue a smaller diameter piece to a larger diameter piece. I opted for gluing separate pieces together as I didn't have access to a lathe. There is also an advantage of this technique, it means the mating surfaces for either gluing, on the bottom, or the o ring seal on the top are 'virgin' cast surfaces that haven't been machined so will be perfectly smooth. I used Tensol 70 acrylic adhesive it acts by polymerising the two mating surfaces together and has a limited gap filling ability it is very strong stuff.