Able to set the maximum size of grid nav,content and aside in v5?

[PascalVorwerk]

In our products, we would like to contain the contents of the 'body' part of the website to a maximum width. This is hard with the new layout structure in v5, because would need to create some kind of 'wrapper' around the 3 grid area's:

    grid-template-areas:
    "header header header"
    "nav content aside"
    "footer footer footer";
    grid-template-columns: auto 1fr auto;
    grid-template-rows: auto 1fr auto;
    

My question is: Is there any way to do this in the current component and if not, how could we make our own component that works similarly but does do what we want.

To clarify again:

Our header can always be the full width

Our 'Body' (e.g. whatever is below the header) should be maximized to a certain width to autogrow from the middle of the screen.

Our footer can always be the full width

the only way i could think of is to create a 'wrapper' grid area around these 3 areas:

.parent {
 grid-template-areas:
 "header"
 "body"
 "footer";
}

.body {
grid-template-areas:
 "nav content aside";
}

but this doesnt really seem to make sense either