Installing island range hood on slanted ceiling

Our cooktop is on an island in the center of the kitchen, and we've never had a vent hood over it, but want to install one since we need a new light fixture there anyway. The ceiling is about 7' above the cooktop, and is slanted/cathedral style (also, the island is at an angle to the ceiling/walls, but I don't think that will matter).
I've seen a lot of installations online and they often involve building a soffit box to get a level surface. I've also seen a few pictures where the chimney seems to go flush with the cathedral ceiling, then has a frame/trim piece above it, which I like the look of.
I know I need a level surface to attach the mounting piece to, but can this be above the ceiling level? I have access to the area above, which is several feet of crawl/attic space under the sloping roof. Can I just put some extra 2x4 support, attach the mounting plate to that, and run the chimney a little through the ceiling? I didn't think the chimney would get hot enough to cause any problems.
Any one have experience with this type of install? I just realized I probably need to have extra chimney then what comes with most hoods. I think I can handle this installation, as it doesn't seem overly technical. Roof is a gentle slope in that area, and I'll use plenty of caulk, since a leaking ceiling would be very unfortunate (wife already raised that issue!).
Thanks for the help!