Stillpoints Ultra SS adapter needed, help...


I just purchased 8 of the Stillpoints Ultra SS for my speakers.
My speakers are the Eggleston Works Andra II. They use a 3/8"-16 thread and the Stillpoints use a 1/4"-20 thread. Stillpoints can provide an adapter but only up to 5/8" long on the 3/8" side (speaker side) and the Andra II's looks to need about 1 1/2" long.

Do any of you know where I can purchase such an adapter or even what its called so I could search the internet?
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Theo, Drilling into the Stillpoints does not seem to be an option. But, I agree with you the 1/4" thread size at the Stillpoints is a concern, my speakers weigh 215 lbs.
I wonder if I should drill a new hole in my speakers near the original made for a 1/4" thread? Then the shaft doesn't need to be too long.
Drilling into the Stillpoints does not seem to be an option

Based on pictures only, it looks like a quality machine shop could easily re-tap the Stillpoints to a 3/8" thread. What's your concern?
Well one would be resale value. Or perhaps it would change the engineered design.
Still that is an option. Thank you.
I respect your concern, but I would drill the Stillpoints before drilling the Andra II. Please keep us updaded on your progress.

As an aside, I am very surprised that Stillpoint does not offer thread size options, especially considering the price!