Well, although I hate PVC for air lines, if you had say a 2" diameter piece that was 20" long, you could place it inside, with one end with a glued on cap and the other with a screw on cap and cast it inside the pipe. Now on the screw end, you need to bolt in a fitting like the casting tanks. Fill the whole pipe, screw the pipe shut and preasure the pipe at low PSI (say 30psi). When you are done you will need to cut it off, but it would work.
DO NOT USE HIGH PSI!! and I would cast this sitting inside a trash can in case it fails! (Put a large bag closed around it)