Had the same problem / issue with my MDD Pro Speakers when I started to use my new PMG5 as my main Mac (still working around to getting to my Mac Pro after 2 years).
Bottom line, it's not worth the effort, you can buy brand new, better speakers for less money than the adapter you are looking for. If you like the form factor of the Pro Speakers, Apple also did USB speakers (with a separate power supply / brick) that may be easier to find. These model speakers came with the G4 Cube, and may have come with other computers, and may have been sold separately.
It may mean you buy a Cube, keep the speakers and resell the Cube.