Hi Mat,
what piece of gear are you trying to connect?
If there are 3,5 socket you can bypass the big din connector and go straight with a simple 3.5 stereo jack
that Boss cable looks pricey and it's not clear if it's a type A or B
every 3,5 stereo minijack will be fine as long as its pins are mapped correctly to the midi A standard:

MIDI 4 (Source) > TRS RING
MIDI 2 (Shield) > TRS SLEEVE
MIDI 5 (Sink) > TRS TIP

you can find all the info you need here:
https://minimidi.world

hope it helps