The real Bob Roberts made some good ones but I don't know if he's still around. I purchased a few from ebay years ago and they are hit or miss.
The real bob roberts one is nowhere near as good as these two. Paid around $70 for mine.
https://www.arcade-projects.com/threads/fs-jamma-extension-harness.12191/
This is also a good one too. It is open sourced so you can build yourself one too. Might be an idea if you ever want to upgrade.
https://www.ebay.com/itm/Custom-Arc...307559?hash=item342bc01367:g:95wAAOSwr~ZfBcN-
https://www.arcade-projects.com/threads/arcade-projects-jamma-extension-cables.7041/
I ended up scooping one of the Real Bob Roberts cables in the mean time. Those cables do look like the next step up, but man, the price is steep. I’ll have to think about it and see how the Bob Roberts cable works out.