Just thinking as I type here Bill. What about priming the inside of each half with epoxy and matt then, while wet, inserting a wetted out matt of carbon wrapped around some kind of bladder/tube that you can inflate while it cures?
Look in the plumbing section at Menards or Home Depot. They have bladders that are used for pushing clogs out of piping. They have a garden hose barb on one end, but you could get/make a converter to hook your compressor up to it. They expand quite a bit and are made to take water pressure from a faucet. Not sure what exactly how much pressure they would hold, but I would expect at least 40 or 50 psi.
