Funny, I just went through this, albeit reversed.

Id bet on the JB weld route. A toothpick or something small like that to get the goop where you need and insert the speedo cable. It should make a nice mold for itself. a bit of lube as mentioned would be good as well to ensure the speedo cable can be removed.

The only other thing I can think of would be 2 small square strips to occupy the space on either side of the speedo cable male end. I think this would be kind of a pain to do though so back to the JB weld.

Gavin