Actually I was thinking the 3 inch as that's closer to the diameter of the fan housing. The wide attachment on the end would mean it would only have to be close and probably not have to be moved/adjusted much.
Is that ~3" the base mount on the Edsyn or the diameter of the intake opening? I was under the impression that would be in the 6 - 8" range, based on the photo you posted.

The 2.5" is a lot harder to find based on my searches (seen it on zorotools.com in the past, but a recent check didn't show it). So I'm not sure where to get it, if it's not part of a vacuum hose kit (hose + intake shroud/nozzle kit that's sold for ~$50 w/ ~3ft of hose). Multiple bases to attach to the other end, including one that ties to PVC plumbing pipe (may just be Jet-On that offers this one).
Another alternative, might be a suitable diameter of aluminum flex duct attached to the intake side, and leave the fan's location fixed. Not sure if it can pull enough vacuum to make this work though (assuming it wouldn't based on what I've seen of the square types).
Perhaps even a squirrel fan & duct (pulls enough vacuum to get the fan off of the bench), similar to one of the considerably more expensive fume extractor systems (box + squirrel fan + filtration material; duct comes off of the intake side). But it's not making use of what you already have (existing fume extractors), and is increasing costs.
