Long chain
pre-covid, I bought a 9 foot Abus 10KS chain and padlock, for locking through two bikes on the rack and through the car's frame. The bike rack came with a medium thickness cable lock, but I only want to use that cable for quick bathroom stops or very low traffic locations. Even thick cables are just too easy to cut.

I don't remember the security chain source, but they stocked Abus and other brands, and sold by the foot. I got the version with a black fabric sleeve with ABUS logos. Haven't needed it so far, hopefully this year.

I used a rope to see how many feet I needed to reach the car's underbody loop. It's enough to easily thread through the bike frames and not have a lot of extra slack near the ground. That helps make it a bit awkward to get good leverage with a big bolt cutter.

I think the chain and padlock weighs almost 20 pounds. I got a cheap Harbor Freight small canvas toolbag to hold it, which works great.

I still wouldn't leave a bike out all night with this chain, but it's good for a restaurant stop, etc.

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Stolen bike racks!
There was a lot of reports of empty bike racks stolen off cars. The nice ones: Yakima, Kuat, etc. The locking pin locks aren't all that strong, and some could be pried loose with just a crowbar.