Revisiting this topic: I don't buy that reasoning - assuming the materials are the same, why wouldn't the 100A rated MIDI fuse have the same fusing element (i.e. the section within the fuse that actually fuses on overcurrent) as a 100A rated MEGA fuse? If anything, I'd expect the larger fuse to...
Here's one example of a connection to the starter busbar/fuseholder: DIY installation tutorial: Enerdrive DC2DC+ charger + Sprinter van + Lithium house battery
You'll also likely need / want to connect to the 'engine running' D+ signal that's present on terminal 1 of the "auxiliary electrical...
For the NCV3, the auxiliary electrical connector (EK1) provides
engine running (D+) (10A)
battery direct (25A)
ignition on (15A)
ref: Sprinter Body & Equipment Guideline (BEG) 2016/2017, section 7.4.7.
The best location to connect a DC-DC charger above ~20A is at the busbar / distribution...