Lock it through the seatbelt mountings, they’re part of the chassis. The car door idea above is very good, stopping both rear doors from being opened.
If the bike is in the back, I always try to park the car with the boot hard up against a wall or tree, anything to make it difficult to open or access. I always cover it fully as well, got a massive tarp which does double duty at keeping the car boot reasonably clean.
As for the bike being covered on insurance, it depends on the policy. My home insurance has an optional extra to cover bikes away from the home, including in cars but there’s the usual disclaimers about it being locked, not visible etc.