They are just shock extenders sold at every auto parts store.

I figure if they are strong enough to be used to jack up a car, it can surly be used for a handle bar riser.

Auto parts store are standard thread on both sides.
And you just need to run a metric 10 / 1.25 tap down the female hole.
Not sure what pitch the teeth are when ya buy it but they are just right where
you can cut the new 10mm teeth nice and no need to drill.
Also helps to keep it straight as you use the old teeth for a guide as ya cut new.
 

The original riser also needs moded.
You will need a metric 10 / 1.25 die to extend the thread down an inch+.
That way it makes full penetration into the lift stud.

A cheap $15-$20 metric tap and die set will work just fine [also very handy], or spend about 5 bucks each.

The original riser end has a hole in it for a cotter pin.
Mine will use a nylon lock nut instead.

In the pic below you can see the stock rubber mounts. Stock risers only really bolt down on the center steel sleeve.
I found they felt squishy some with the extra leverage.
So I used a hogged out washer so the risers by passed the center steel sleeve and bolt down to the head stock but I didn't use on on the other side.
Still dampens vibes some but that top washer doesn't let them rock around with the leverage so much.

The bars feel strong and secure. The rise is nice and comfy.
This is a mod that the novice might want to get help with.
Taping the bolts longer is easy but you will also need to make two holes in the Ign cover.
Once flipped over it is easy to see where the caps sit to find the right spot.
I would suggest a hole saw and going slow, so not to make it look chopped.