From what I've heard, Hogg islands can sometimes get above 6 feet. You might try a Sonoran Desert (Mexican) BCI, Crawl Cay (expensive, but smaller) or Cay Caulker Boa. Ben Siegel Reptiles has some Cay Caulkers for sale right now I think. I've got a few Sonorans, but they aren't ready to go yet.
All 3 get about 5' and I've never seen one get above 6'.