There's a lot out there about electrical adapters to use an ATX PSU with AT boards, but I'm looking for something else and having trouble finding it - not helped by the fact that "at" is a normal English word and a lot of people use 'large', 'big' or 'full' inaccurately to describe overspecifying their components, resulting in a lot of spurious searches....

In ye olden days, power supplies were bigger than they are now. Originally the PC and clones had a huge power supply with intregrated switch sticking out the side of the case. Later the same large form factor was used without the integrated switch, particularly in early tower model systems:

These were used in the late 1980s and started disappearing in the early 1990s as the current physical format PSUs (originally from the LPX standard) came into use. Unfortunately said power supplies are very uncommon now, and if you find one it's at least 35 years old with the issues that brings with it.

The mounting holes are clearly different, at best you could get two screws into a modern PSU. Now, with some bodging and drilling it should be possible to make something fit, but given my metalworking skills it would look awful, so if someone can point me to a ready-made physical adapter to put a regular-sized PSU into a case like this, I'd be very grateful.