Generally, a 42 inch hood width will suffice for a 36 inch range, so long as the hood is not more than about 36 inches above the range and is sufficiently deep, front-to-back. If the usual residential hood suppliers do not have a size you want, custom shops like Modernaire will be willing to step in. An insert is a hood without a fancy exterior, but the intake aperture size requirements still apply.

Use the information dispersed in the threads on this forum to do actual sizing of flow rate and MUA requirements. Many are fairly recent.