If you are partial to the old Jenn Air downdraft ranges (1970's-1990's) AND you want to install them in an island or on an inside wall...Jenn Air made something for you.  The standard "downdraft" idea involves an exhaust vent to the outside of your house.  However, Jenn Air came out with a few models that allowed the venting to pass through an electrostatic air cleaner (much like a smoke-eater or newer breed of Ionic Breezes) that was mounted below the stove and vented back into your kitchen.  Models like the old F120 and F220 come to mind that seemed almost identical to the old D120 unit but with the EAC (Electronic Air Cleaner) mounted below....and possibly a microwave oven mounted overhead.  So how does this help you?  We'll, they're probably pretty rare to find anymore but if you do, it could quickly solve your problem of mounting your old downdraft stove into an island or inside wall.  I've stripped down many Jenn Airs over the past 7 years and have only come across 1 unit like this. I originally kept the whole EAC unit but now I've only got the replaceable cartridge....waiting for that one other person who knows of these units!  Well, at least you know there is something out there...now your fun begins in searching for the burried treasure....good luck...I'd start digging on Ebay!