For a data struct like this I like to use the `Struct` class, as it's nice to use dot syntax and get `NoMethodError` for typos rather than `nil`:

``` ruby
Slot = Struct.new(:worker_id, :sandbox) # this would be in the main body of the class def, obviously
slot = Slot.new(i, nil)
```

That said, the sandbox part isn't used, so should it just store the `worker_id` directly?  Although, maybe the `Slot` is better as it can expand easily for other metadata.  Not sure I like the `Slot` name, though.