The TP2000 counterlungs are made of 5" diameter motorcycle tire. Note the lack of breathing hoses - I simply put the mouthpiece directly between the two counterlungs to form a continuous loop. The mouthpiece is an off-the-shelf rebreather mouthpiece with one-way flapper valves, which I didn't want to screw around with trying to build from scratch.
The scrubber cannister at the bottom is a heavy-duty acrylic cylinder, which is nice, because you can simply glance down at any time and check the state of your scrubber. That's a standard inflator hose that's been modified to include a PO2 readout (the sensor is inside the right counterlung, of course). You hook either O2 or diluent up to the inflator for injection. This seemed OK, because it's easy enough to swap off the LP hoses, and you don't usually need both at the same time.