Unfortunately the 2.5mm gap left between the inner tube and outer tube is too narrow to insert a solid rod of reasonable diameter, which would be tapped at both ends with an M3 or M2.5 thread bore. Such a rod would need to be at least 6mm in diameter. One solution is to do away with the inner tube and replace it with an epoxy moulding like this:
The problem then becomes, how to make a moulding to form this part. The outside of the mould can be formed using the outer tube, while the inside can be formed using the outside diameter of a tube diameter which corresponds to the diameter of the cell, though slightly wider.
Next, there would need to be a base upon which the epoxy could set, such as a disk of plastic glued in place, though sufficiently brittle to remove after the resin has set. The inner tubing would have to be supported to occupy the centre. This could be done by glueing it to the disk.