Good stuff. Plenty of very succesful and reliable model subs have been built on the old Darnell box, but I'm personally more a fan of cylinders.
If you have access to a lathe, then making the endcaps for a cylinder should be straightforward.
Material wise you can use PVC or acrylic pipe. The latter offers a see-through cylinder, but it is more expensive and fragile- if you drop in something hard it will most likely crack or at least chip, whereas PVC will bounce. In the States, Polycarbonate is very popular. Like acrylic it's transparent, but is much tougher, in fact, AFAIK it's the strongest thermoplastic. Unfortunately it's very expensive here in the UK, especially for the larger diameters which is why acrylic is more popular. It gets the job done, although not a good idea to use it with gas systems.
PVC pipe is readily available in various sizes. A large boat like the Type VII will easily accomodate 110mm diameter pipe, I often see lots of this chucked out in skips, but you can also purchase a length new for very little. Endcaps can be machined from sheet plastics like ABS, acrylic, polycarbonate or you can even machine them from materials like aluminium. I would avoid wood, as it's difficult to seal well and water always seems to find a route in somewhere or other.
Shaft seals can be turned on the lathe, or you can purchase ready made. They're not ultra expensive, so if you're unsure about making those vital components, it's worth looking at the commercial units which work very well.