Sticking with the theme of MD you would naturally want to go internal, for the purist mind anything external is a compromise.
Having said that I am not a purist and have two MD with the GLR slimline retainers and love them. But I also have two MD with the aeropack retainers, one 38mm, and one 54mm. The issue with these retainers is the inflexibility of where they are mounted vs the motor hardware. In my 54mm MD CF Tomach I am using the internal aeropack matched with Loki 54/2800 hardware and just drilling countersunk holes to mount the aeropack. This way I can remove it for maintenance. If I need to drill more holes then so be it I can always use a filler ring screwed into any holes above the hardware if using shorter hardware. I originally bought all of the extensions and have the cases and even motor blanks in the case of CTI. The idea was to use combinations of extensions to mount different hardware. But I abandoned this idea after I mocked it up and felt there was too much of a trade off with the extra dead weight and issues with using combinations of extensions.