We will be ordering something very similar from Metastone. Several of the appliance stores in the area have them on display. We are looking at Abbacu and Modern Air for inserts. Metastone sent me a picture you might find helpful:

http://picasaweb.google.com/slkayser/PebbleDrawings#5243115860134943170
http://picasaweb.google.com/slkayser/PebbleDrawings#5243115863786088706

An interesting question. We had something of the same issue when we installed our liner in a range alcove. If the size of the opening of the actual hood will be much larger than the size of the liner, then recessing the liner may actually allow the remainder of the hood surround to function as a secondary capture area, allowing greater holding time of fumes awaiting eventual evacuation via the blower. Ideally you'd want the area under the hood outside the actual liner to be closed off flush with the bottom of the liner. This would avoid creating deep recesses outside the liner that would tend to trap grease and smoke.

A much better option is to use a hood liner that is custom sized to completely fill the hood space. I've posted a bunch about my experience with our Modern Aire liner. It's a fabulous unit, which we custom sized to 64'' wide to more fully cover the range (which is 55 1/2''). The custom upcharge is extremely small and with it they will build to any width or depth, and can offer shallow or deeper hoods. They also custom locate the duct connection, electrical connections, and can do remote infinitely variable speed control for a remote blower.

Using a custom liner means that you'll have maximum ventilation effectiveness. Additionally you can mount it flush, and your lighting will also be closer to the range, which is nice.

So, clearly the optimal solution is to custom size the largest liner that you can fit, making sure your blower is sized to handle it. I am also a huge fan of remote blowers based on the enormously lower noise level from our 1266 cfm Fantech remote blower compared to the 600 cfm VAH hood we have at the lake.