I looked and looked for exactly what you are looking for for my Checkmate case but never had any luck. I know they used to make such a device as it's still listed on Amigakit's website but hasn't been in stock for a long time.
https://amigakit.amiga.store/product_info.php?products_id=485You could possibly post on Amibay what you are looking for and someone may be willing to part with theirs.
I ended up just making a custom cable to run from my DVD drive to the back of my case and used this mixer to mix them together.
https://www.amazon.com/gp/product/B00102ZN40Works like a charm.
