I just compared Dimensions of the Two cameras, and they seem to match surprisingly well on Paper from a brief look at it.
BMPCC 4K Website says, its WxHxD 7"x3,8"x3,4" which is 177,8mm x 96,5mm x 86,4mm.
The Chronos 2.1 Mechanical Drawing says, it is WxHxD 155mm x 96mm x 67,25mm.
So from that alone, anything that will fit an BMPCC 4K should also have room to fit an 2.1 in it (unless i got my math wrong or missing something obvious).
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BUT, the BMPCC 4K is some kind of tilted forward Shape, and most Images i found from an Quick Image Search for "tilta BMPCC rig" show Cages, which most likely will either block the Power Button, Trigger Button, SD-Card Slot; and/ or the HDMI Out or other I/O on the Side of the Camera. Also watch out for Ventilation, some of the Cages might as well block considerable amounts of Ventilation Area and the Battery compartment.
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So as a Summary, would sure be worth a Try because the Physical Size is very similar, but watch out that whatever Cage/ rig you end up going with wont block any IO / Buttons/ Slots on the Camera you might want to use later. I am personally not aware of any cage that is currently for sale, which would be a known good fit for a Chronos Camera, thats why i started to build myself a custom one (work in Progress; see:
https://forum.krontech.ca/index.php?topic=683.0) for my personal needs.
If you find one that actually fits well, i would also be interested to know!