If you have a better suggestion, I'm all ears. But I settled on that because A) it's simple, B) it's short, C) it's something that does actually come up in practice, so it's not too contrived.

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"What's the difference between w/uniq and w/box? Both seem to create boxes."

Well, to explain this... unique variables are actually implemented as anonymous boxes. So implementation-wise, there's not much difference.

The difference is in the actual macros "w/box" and "w/uniq". Here's the definition for them:

  $mac w/box -> @args body
    'w/new-scope
       | var ,@args
       | body

  $mac w/uniq -> @args body
    'w/box ,@(args.map -> x 'x = make-uniq;)
       body

  w/box foo = 5
    foo

  w/new-scope
    | var foo = 5
    | foo

  w/uniq foo
    foo

  w/box foo = make-uniq;
    foo

  w/new-scope
    | var foo = make-uniq;
    | foo

They serve exactly the same purpose as "let" and "w/uniq" in Arc.

If you're asking why you can't just do this:

  $mac if-box ->
    w/box temp
      'w/box temp = 5
         if temp
           temp + 1

  $mac foo ->
    w/box bar = 5
      'bar + 2

I thought about having it throw an error for local symbols, which would have required you to write the macro like this:

  $mac foo ->
    w/box bar = 5
      ',bar + 2

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"I tried running this and got the following"

It works fine for me. And in fact, the code snippet you posted seems exactly the same as what is already in the tutorial. Maybe you intended to paste something else?