Combiner boxes typically have a DIN rail for breakers or fuses, so that would be inside the box.
Sometimes pre-assembled, often sold empty to be configured as needed.
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MC4 is typically rated 30A, and some "Y" cables are made with 8 awg. Unless it had a special MC4 connector rated 55A, and you were sure the mating cable also had a compatible MC4 connector of same rating, couldn't rely on MC4 to carry the combined current.
I think if you got a 4-Y cable, or three 2-Y cables, you could just cut off the MC-4 connector which would have had to carry the combined current and splice it to the home-run wire. Put four MC-4 fuse holders on the Y cable and connect to PV panels.
MC4 fuse holders aren't touch-safe but the MC4 connectors are, so disconnect both MC4 cables before opening. Interrupt current flow before disconnecting, so not under load.
I think I've heard that new/future requirements are that PV breakers be non-polar? If so, that solves my concern that the polarized ones can't actually provide the protection they were supposed to. A 4-pole breaker (all poles ganged) would work fine even if polarized. Fuses are good, but don't serve as a disconnect under load. Four touch-safe fuse holders and a breaker as disconnect would work, or shut off charge controller by opening breaker on battery side before opening fuse holders.
This is the one I wouldn't trust unless the breakers were non-polarized, or had a wire slipped through the handles to gang them. I haven't heard of any faults that they should have interrupted and didn't, just expressing my opinion/analysis of the breaker ratings and required performance.
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Prices on those all seem high, but vary from difference sources. It is the breakers and fuses themselves which particularly need to have the right voltage/current/polarization specs.