Replace old double built under oven

Normally for single built under ovens, the housing has a shelf pack which the oven sits on, this raises the oven up to the correct installation height. This then leaves a void underneath which will need to be covered with a plinth which matches the existing unit plinths. I would guess that as you've already had an oven but are replacing it with a smaller one then you won't still have the shelf pack but you can easily make one from melamine covered board which is readily available from places like B & Q.