Here is a simple flexible beam coupling I made for 6mm shafts. It uses split brass inserts for a completely no-mar grip on the shafts.
The body is ½" dia. aluminium. Flexibility is provided by alternating saw cuts which pass nearly completely through the material, leaving a small web in the middle. There is a 6.5mm hole running the length of the coupling, and the ends are bored out to 7mm to accept the split brass inserts. All parts were turned to ensure concentricity.
To make sure the bores on both ends were aligned, they were machined as follows. First, one end was machined fully (bored out to 7mm , chamfered etc.). Then, with the stock material still in the chuck, a light cut was taken where the other end would be - this gives a cylindrical surface concentric with the bore we've just machined. It's then a simple matter to cut it off, mount it in the 4-jaw, and center it exactly and bore out the other end.
It has excellent grip, thanks to the clamping effect of the split inserts, and seems to allow for a few degrees of misalignment.