I'd say that you're somewhat in luck, Tomm. The Sonaro was originally a combo amp with a single 15" JBL speaker. Later, it was split into a head/cabinet unit, still with a single 15", so a refrigerator-sized cabinet wouldn't be as authentic. On the down side, original cabinets are pretty hard to find. But if you're thinking of going custom built, that's not an issue.
I think that the original Sonaro cabinet (after they separated them) was 29x24x11.5. Later, in the early 70s, Sunn came out with the 115S, which I think was 24x24x15. The 2x12" Solarus cabinet is also 29x24x11.5, and is open back. Suitable for guitar, not so much for bass.