Yes, you do need to make a bevel along each edge so that the boards join up like the sides of a polygon when seen from the end. A full size drawing will be a big help.
It's a good exercise in accurate hand planing, or else you can make a jig of some sort to reduce the errors. One way would be to make a shooting board with the bed at a suitable angle so your plane would still run as usual. Another way would be to make a sloping base for an electric router.
You'll need to decide whether you want flat facets to the lid - which is easiest - or a smooth curve. If you want a smooth curve you will need to use thicker wood so that you can plane the angles away.
If you want it to be really complicated you could make the whole lid taper in the length so that the top is like a slice off a barrel - but that could be quite challenging!