There are 5/8″ to 3.5″ adapters out there, pretty simple so not much to differentiate them. They seem expensive for what they are made of, but they would be a low quantity item to produce and stock.
I made a temporary one out of a 3.5″ plastic plumber’s plug with a hole drilled in the middle and ran a 5/8-11 tap through it. Not something to hold up with frequent use but good enough to play with sun shots with the K&E.
The tap ham let me rig several items to play with.
A product I haven’t seen would be a low-profile tribrach adapter, 3 point mount on the bottom, 3.5″ on top.
If you left the leveling screws on the transit all set the same height and centered, and did the adjustment with the tribrach and plummet you should be centered as well as with a plumb Bob in a light breeze.
Putting together a 3-point to 5/8 tribrach adapter and a common 5/8 to 3.5″ adapter might stack up high enough to cause problems.
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