[plt-scheme] What's the best thing for a distinct handle?

I'm using distinct values, compared using eq?, as handles (because
they're unforgable and I can control who gets them).  I'm wondering what
is the best type of MzScheme value to use for this.  I figure whatever
would use the least memory is the best, so what type of MzScheme value,
that can be used for this, uses the least memory?  A box of `null'?  A
new empty string?  A new empty vector?

Thanks!

-- 
: Derick
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