I recommend getting a drawing from the source of the insert and any cabinet directions that they may have already published.

Care must be taken to ensure combustibles (which includes a lot more materials than one might imagine) meet the distance requirements specified by your stove/range/cooktop manufacturer.

Normally inserts are surrounded by cabinet matching wood, so it may be a good idea to have the insert's lower flange flare out under the wood. That is, have the wood tight to the insert, but the entirety high enough to meet the combustion distance requirements.

You may need metal stud framing for the form. You have to print and understand the specs of the insert and the range before being able to design anything. There is no universal this is how you do it. But there are a LOT of bad images online that should be labeled What Not To Do instead of Pinned.