The handle of the staff is made from 1" polycarbonate tubing. Make sure the interior diameter of your tubing is at least 7/8", so the battery will fit inside.
A note about polycarbonate vs. acrylic:
Acrylic tubes are cheaper and a bit easier to find, and they seem basically the same, but acrylic plastic is a more delicate material and is much more likely to shatter. This might not be an issue if you're a careful wizard who never lets anyone else play with her staff, but I'd really advise spending a few extra dollars to get polycarbonate. Magic tools should be sturdy.
I get my polycarbonate tubes from FlowToys. You can get a wonderful end cap for the bottom from these guys as well. Capping the bottom end will extend the life of your staff and keep mud and debris out and away from your LEDs.
I am using a size 12F, which is about 5' long. Once I add the water bottle to the top of the staff, the whole thing becomes almost 6' long, just slightly taller than me.
Remove any labels from your water bottle using Goo Gone or another solvent. Cut off the top of the bottle just below the threads, so that it fits snugly over your polycarbonate tube. Secure it tightly with gaffer's tape or duct tape. You can add some tape around the top of the tube as well to get a nice tight fit.
Put some plastic sheeting down on your worktable to protect it from expanding foam and paint splatters. Expanding foam sticks to just about everything and does NOT come off, so you'll want to wear gloves and an apron or coveralls as well, or you'll ruin your favorite yoga pants.
I'm using Great Stuff brand expanding foam, the Gaps & Cracks variety, since it expands a bit less than the Big Gap Filler variety so I can get finer details. Mind, it's still tough to get fine details with this stuff. It goes where it will, so it's great for messy organic shapes, but if you're looking to make an Elven High Court type staff, this might not be the best material for symmetry and smooth lines.
I sketched out a rough design on the water bottle with a sharpie before adding the foam. I want the water bottle to be covered enough that it doesn't look like a water bottle anymore, but have large "windows" to let the light shine through. Also, the beads of foam need enough contact with each other that they'll support the bottle without breaking off.
We'll need to cut through one of these windows later on to get our electronics inside, so keep that in mind when deciding on your design.
I also placed the end cap on the bottom of the staff, with a piece of saran wrap between the cap and the staff. This way the foam will flare out at the base of the staff, leaving room for the end cap inside, but not fusing to the end cap itself.
(I'm not sure if this was necessary -- I have no idea if the foam will stick to the silicone cap or not -- but I didn't want to take the chance, because this foam seems to stick to everything!)
Shake the can and attach the straw to the top. It's not a bad idea to practice on a piece of scrap wood first, to get the hang of how the foam comes out of the can, so you can get smoother lines. Once you've got the hang of it, start covering your staff with foam in long vertical lines.
Misting the foam with water will speed up the curing time from 45 minutes to around 20 minutes, so having a spray bottle handy is a good idea.
I found it works best to cover the top half of the tube, let the foam set up, then flip the staff and do the other side.
In between sections, close and cover the tip of the expanding foam straw with tape to keep it from oozing out or drying up.
You'll want to cover the tube with enough foam that it makes a cohesive casing. The foam is very strong when it's stuck to itself, but loose, open strands will be weaker and a bit more likely to break off. At the same time, it's fine to leave a few gaps here and there for more light to shine through.
Once you're happy with your basic shape and things are all covered, let the foam cure for at least 24 hours.
Once your foam is fully cured, grab your keyhole saw or bread knife and get carving.
If you like the cartoony look of the foam you can skip this step, but I really like the carved-down look. It looks like a tree branch to me, and has a lot of interesting organic texture to it.
Carve lengthwise along the staff, just taking the top off the foam and roughing out your shape. You can always add more foam if you take off too much.