After asking on a forum, I did what follows.
I found a 10/16A multiple socket outlet laying around. Italian AC socket outlet has three inline holes and the 16A slot fits smoothly a banana socket. Some modification was needed, but the annoying mechanical part was already done.
After disassembling the plastic case I analyzed what was inside. On the cover there was the safety mechanism that prevents someone from inserting only one pin into the socket. It is made of the blue things and spring you see in the picture above. They come off easily. Then the surprise: current is distributed with these "rails" and the connection to the plug
Since banana plugs come with two bolts, I used the first to secure the banana to the top cover, and the second for electrical contact to the rail, as visible here:
Then I soldered all connections for a longer life without need for servicing. I needed the 30W iron and a bit of patience, but it was easy. Just be careful not to overheat and melt the plastic structure. Also the supply wire <-> rail connection has been soldered.
Now I have got 4x 12V sockets just behind the PSU for my radioes.
Apparently only Italian AC accessories are so easy to adapt for this use. Usually the cheapest model adapts best. In fact I had bought a new one for few €, but chose to sacrifice this one.
