Flat pieces of wood to fill the area...I have a 54" hood over a 36" cooktop + a 12 " Miele deep fat fryer. There is space between so this = the exact width of all the cooking surfaces. It is not necessary to have the extra width on either side if you have a powerful and efficient hood. You can have flat pieces of the cabinet wood to fill the area for 3 " on each side. It can be painted to match the cabs. My wood hood surround is exactly as it was almost 3 yrs ago after extensive use of high heat cooking. Don't panic....you don't need that big of a hood and I wouldn't accept it. Hope others chime in with their info. Here is a pic of ours...

As previously noted in many past threads, one can have at best two of the three characteristics: performance, aesthetics, and "not overly pricey." One can also trade off among these for some balanced result. But in no case will a hood or hood insert suitable for a 48-inch range that will actually be used for cooking not be pricey by most observers' senses of valuation. One should be aware of not only the insert's and exhaust blower's costs, but also the necessity for make-up air (MUA). Supply of MUA in many or most cases requires some type of MUA system, with its own ducting, blower, control system, and perhaps filter. In this Appliance section of Houzz there are many hood threads developed over the past 10 years with information related to manufacturers, and more importantly for beginners, information related to requirements, particularly performance requirements, but also approaches to keeping down noise. Rather than repeat all these here with all the necessary background information, please take some time to search these out and review them. Then specific answers can be given without requiring an extensive level of commentary. This should also clarify the scope of information you will have to provide to refine the analysis.

I think it really depends. Nothing that moves that much air is going to be quiet. If you can have remote blowers and inline mufflers, then you can reduce some of the sound; and in that case, a VAH might very well the obviously inferior choice. But even then, you still have the noise at the baffles. So, don't expect the hood to be whisper quiet. We have a 20 odd year old VAH. As long as the vent pipe is adequately sized, it works great and noise level is IMHO in the expected range. We also have a new ModernAire hood with internal blowers. It has more power than our old VAH, but it can be really really loud. And part of that is simply the whistling sound that the air makes when it passes the baffles at full air velocity. If I turn down fan speed, it's about as loud as the old VAH. In other words, I don't really perceive a big difference between either designs. Pick what works best considering all your other design constraints. And absolutely make sure that your vent pipe is as large as you can make it.

Not saying it couldn't have been done better to fit the hood liner she wanted. But you need to roll with the punches and make things work when remodeling. Her custom wood hood is already being made or finished already. If this was a rectangular hood, there wouldn't be a problem. So...You find a blower package that will fit and follow the manufacturer recommendations. The CFMs and the height of the hood from the range. Those are what's important, One more thing I think that people don't realize. The blower width does not need to be the exact width of the range....the capture area does though (or wider). Would it be better if the blower could be wider? Sure....but in the long run, the 48" will work fine as long as the CFMs are high enough. If money isn't an option, build a bigger hood. If it is....then make it work to manufacturer specifications.