Not an arboreal, but on your want list and quite handleable: Black Milksnake. I have a breeding pair, and they're quite enjoyable captives. I keep mine in 48 x 24 x 12" enclosures, but an extra foot or two of height would be well used.
Rainbow boas are another arboreal-ish possibility -- I keep mine in 48 x 24 x 18", but again more height would be used.
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