You need an insert.

"Need" an insert because that is how they were designed to be used? Or because its unsafe to use? Or because it won't function correctly?

Because a liner is designed to be inserted inside a wood hood, with all of tbe correct support and attachment points hidden where tbey won't attract nasty grease and dirt, and the electrical components are designed to work inside that wood surround. Wood above a range is a fire safety issue, and it MUST be properly covered by a metal liner designed to do so. Also, a wood hood and insert must be larger than the burners that it covers below to meet fire codes. Standard under cabinet metal range hoods are not designed to be inserts.