Update: Monitoring heat at the fuse and connections with this setup for a few days now. Increased charge and discharge current up to 120A (around 6600W at charging voltage). No appreciable heat buildup, only slightly warm to the touch after a couple hours charging.In the meantime, while waiting on the blue sea holder and a different fuse, I've modified my spare according to some recommendations here.
Removed the thin metal extension pieces, added a bolt to the top so plastic isn't in the compression path, and bolted my lugs directly to the fuse legs (tinned copper to tinned copper). I didn't have a lock nut of the correct size but I should get the new holder before thermal cycling loosens it. I torqued it together pretty tightly. I'm still throttling current on this setup.
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Did get my blue sea fuse holder but still waiting on a new fuse to fit the holder.