In Mark Mervine's review of the Maerklin/Trix Sggrss 80 Container cars, I posted some information that was emailed to my by Rocky Rail (Belgium).
Apparently the Portuguese company PT Trains, which had previously released some HO scale containers has now released two different sets of adapters, which can be inserted into the sockets on Maerklin and other container cars, which allow the use of Herpa and other containers which had sockets instead of studs on the underside of the containers.

https://www.marklin-user...Container-Cars--A-Review

Each set comes with 24 adapter pieces, which can be used to completely outfit a 60 foot Sgns container car. As many containers only have the four sockets at the extremities, you would only need to use the 4 which apply to your particular container type. If you have a single 45', you need only 4. If you have 2 30' containers, you would need 8, same with a 40' and 20' combination. For 3 20', you would need 12.

If using Fleischmann type containers, you could have all of the sockets filled, as the Fleischmann type containers have multiple sockets underneath).

Regards

Mike C