Quick update.
This is the adapter I got:
https://www.ebay.co.uk/itm/182240547207
Luckily, it was plug and play. The only part not matching was the old radio's fascia. The opening is too narrow. I fit the new radio into the slot without the fascia. I looks ok, but there's a slight gap left, so will have to 3d print something in to fit in there.
Other than that, the radio works great. The bass seems a bit quiet, but it's not too bad. I got a Blaupunkt Skagen and probably will fit in an under-the-seat sub.