The theory of operation of the Berg shifter shortener is pretty simple.
They move the fulcrum of the lever to shiftrod by elevating the original
shifting pivot using an aluminum spacer.  Adding a retainer to retain the
original shifter spring a bit farther up the lever provides the same "feel"
and spring loading as the original.  Take off your rubber shifter cover and
take a look at how it works now.  Now imagine raising that plate up say 0.75
inches and re positioning the spring retainer.  That's pretty much all it
does. So to replicate the mechanism make a spacer, spring retainer, and add
longer bolts.

I put mine on so long ago I've forgotten the full details but is is
essential that you maintain the relative position of the shifter plate so
mark it's outline with a pencil so you can re-position it after installing
the spacer.

Note how far up the shaft the original spring is retained and add the spacer
thickness to the original spring location to determine the new retainer
position.  Berg has a set screw retained ring on the shaft to position the
spring.  This design could be improved upon as the retainer needs only hold
the spring in place.  The spacer itself could be made from almost any
incompressible material.  Oil resistant plastic, metal, teak, rosewood,
depleted uranium, dinosaur bone, whatever you have around the shop.  The
retainer could be a clamp of most any sort.  Using ones fertile imagination
and handy bits and pieces.  You'll need longer bolts by the thickness of the
spacer.

Or buy one.  Copy the section of the spacer.  Figure out a clamping system.
Voila'

p