This insert just sits under original mount, restoring the height they loose to sag over the years, and has wings that match the taper and slot in the OE mounts for both location and to stiffen the mounting somewhat to reduce fore-aft movement on the engine under power, etc.
Old tired mount:

This should just kiss the mounting to prevent upward movement and reduce fore/aft - but over time the mounts sag under the weight of the box - as you can see from the gap - obviously this is even larger when on the car.
Insert is pretty simple, the lower is designed to simulate ~60A shore to match the OE rubber, the taper and wings that run up either side are 90A+ to reduce movement in both old and new mounts to tighten things up.

Just presses over the collar:

And then fit the mount as normal, et voila, gap taken up - with the gearbox weight on it it should result in being back to just kissing the face with a little preload:

And of course, the wings and the taper reduce movement fore/aft even on a new OE mount.
Fairly simple, quick and easy to fit.
£12.50 including UK postage