Les,
When I first got my Dragonfly, I had a similar problem in that the throttle slider was in 2 pieces - they were made of a zinc based alloy and were prone to disintegrating with age. I made a replacement from phosphor bronze which is still working well (43 years on!). I'm afraid that, without dismantling mine, I cant come up with any dimensions, but you should be able to work them out from the mating parts that you have. From memory, the block is a shallow inverted 'T' shape that is curved to fit the diameter of the 'bars - the centre being angled to match the helix in the twistgrip sleeve. The centre of the block has a square hole to accept the 1/8" x1/8" cable nipple, and also about a 1/16" wide slot running out one end for the actual cable.
I found it was better to fit the twistgrip with the cable at the top - with the cable underneath, water got in and settled around the slider, causing it to be very stiff to operate (probably also hastening the demise of the original slider!).
Hope this helps,
Regards,
Eddie.