I'm planning to install new kitchen cabinets. I'll be covering up the kitchen's existing sidewall diffuser with new cabinets, so I'd like to relocate it to a new register in the toe kick of the sink cabinet, on the other side of the kitchen. I'm free to cut whatever holes in the floor, walls and cabinets are necessary and I can fit some combination of duct transitions both inside the crawlspace and underneath the cabinet.
How can I make the transition between the 5" flex duct in the crawlspace and the ~4" high toe kick in the new cabinets? The new cabinets will be the type that are on leveling legs, where the toe kick is installed separately. This precludes options that use the underside of the cabinet as a plenum.