Why stop at C90s though but? I used to use C180s in my GF-555 back in the day and sometimes, it would play the whole of one side of the first one before snapping the second a few minutes after the mechanism clunked its way over to deck two. If I tried that with it now, it would almost certainly make a call to its union rep and have me banned from owning any more classic boomboxes for ever.
I do love the look of these long boxes with their three decks, they look like some bizarre sort of radio-sausage hybrid.