I don't know anything about Grayshift boxes, but you can pair-lock your iPhone so it can't be accessed by devices that connect to it physically.[0] Note that there are some downsides like not being able to restore from a backup from a date before you pair-locked the device, and only being able to backup and restore from that device. (Better hope it never dies!) But if you've got a burner phone you don't want authorities to be able to get into (unless they take the device it's locked to, as well), it's a workable solution.
[0]https://www.zdziarski.com/blog/?p=2589