There's a thread about ductless hoods on the Appliances forum, first page right now, that you might find interesting.

line the cabinet to the right up with the blocked spot above. It's ok to give exposed hoods some room on sides. I actually prefer this to give a more open look & keep the occasional heavy steam that did not get sucked up off the surrounding cabinets. same sizes on both sides for symmetry. Tile all the way up. Read the stove and hood instruction manuals for suggested placement. It also depends how much you cook, in what is the best type of hood & the spacing needed. Here's an example, that explains why.. From Proline Range Hoods FAQ section: "The third variable which affects the effective capture area of your range hood is the size of the hood. You can have the strongest and most powerful vent hood, with the best design, and if you don’t cover the cooktop and trap the cooking exhaust you can never get rid of it. This is especially true for range hoods over island cooktops. Once you have considered your cooking style consider the size of the range hood. Try to size the hood 6” larger in width than the cook top. Most range hoods come in 22-27” depths with some of the range hoods being 30” deep. The more coverage the better for the serious cooks. If you use your cooktop and you are serious about cooking then remember “coverage is king” especially in an island range hood setting. You can also help yourself by trying to keep your cooking over the primary capture area of your range hood. BBQ and Heavy Smoke cooking environments, both indoor and outside need coverage so size of the range hood is critical. Again, if you can catch the smoke and exhaust you can get rid of it but if you don’t “trap” or catch the smoke it is almost impossible to get rid of without removing an inefficient unnecessary amount of additional air in the room. All of which must be heated or air conditioned in most cases." http://www.prolinerangehoods.com/1-801-973-3959/support/common-questions/

@artemis78: "Our cabinets are 24" deep and we're not planning to change them, so sounds like we do need to stick to freestanding if slide-in is that deep." There's not a uniform definition of "depth" among appliance manufacturers. Examples: Frigidaire GCRI3058AF: Depth including door: 25-3/4", Depth including handle: 29-1/4" Samsung NE63B8211SS: Depth including door: 26-7/16", Depth including handle: 28-11/16" LG LSE4616ST/LSE4617ST: Depth without door: 24", Depth including handle: 28-15/16" So the Frigidaire (the only induction freestanding range available) is actually the deepest of the four ranges listed, though they're all within 1/2" of one another.

"Can we attach flexible piping to the 6" duct" DANGER. You cannot install flexible pipe for a cooktop exhaust. Flexible duct is prohibited by Building Code. It is a fire hazard. Here is why: the duct must be smooth so that grease particles do not get trapped and cause a fire. If you have grease particles in the crannies of flexible duct above your hood, and your cooking has even the tiniest flare-up, that grease will catch, the fire will whoosh up the duct assisted by the pull of the air flow, and then spread across your walls, and ultimately your house. You may be thinking "this isn't likely for us". Except that according to the Insurance Information Institute, a home fire occurs every 87 seconds in the U.S., and 50% of all home fires are caused by cooking fires. So just use rigid metal duct, and an elbow if you need to turn it in a different direction. "Will it affect the venting power?" What will affect the venting power is the 6" duct size you specified. The duct needs to be a 8" or 10" diameter for good air flow to pull up the smoke and grease. People get by on a 6" duct size because their homes already have the 6" duct behind walls so they are stuck with it. But if you are installing new duct - which it sounds like you are - you have the opportunity to install 8" or 10" duct. It does not make much sense to spend money on a new hood, but curtail its air flow from the getgo with a 6" duct. "induction doesn't cause as much smoke." Not true. The smoke and grease is a function of what you are cooking, not the energy source. If you are cooking bacon, frying a hamburger, or stir frying, the smoke and grease are the same whether using induction or a gas cooktop. If your family is vegetarian, or you never use the cooktop for frying , then it makes sense to say your cooking does not cause as much smoke, but it's not because of an induction cooktop. I think it's best to budget for smoke and grease, and if you don't often generate it, you can turn the hood fan to a lower speed, but on the occasion you are cooking a smokey item, you have enough ventilation not to set off the smoke alarm. Not what you asked, but essential measurements: - width of the hood is ideally 6" wider than the cooktop, so your hood should be 42" wide. - depth of the hood front-to-back should be 24" in order to cover the front burners. This is where a lot of cabinet hood surrounds fall short. You can get away with a 22" depth, but not less, or you would have spent all the money and effort for a hood that does not even cover your front burners. - Distance from cooktop to bottom of hood shoud be between 30"-36". Shorter than that and you will have a cramped feeling when cooking. Higher than that and you are giving the smoke and grease too much opportunity to escape.