The closest to a "nested" option based on how the current event tool works would be the Items, with their options. Have a car set as an item, make their options the time slots.
As an example, one of your items could have a setup of:
name: Chevy Bolt
max quantity: 12 (for your available time slots for this vehicle)
max order per registrant: 1
options: add 12 of them, each with a max quantity of 1, and name them for the time slot they represent.
The only issue is this wouldn't necessarily prevent a single registrant from potentially securing a time slot for every single car model. If you're okay with this, then there's nothing to really address. If you want to discourage registrants from doing this, then I'd advise referring to the custom text bit in my earlier response.
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