Please consider using a "ticking stick" that is used by shipwrights so outline irregular shapes such as v-berths. Very easy and on a nice day fast easy and fun. You can tape a piece of butcher paper or I use last years Christmas paper to on the white side. "Tick" out each corner and a few marks on the curved hull.
You then tape up a full sheet with he same paper on your driveway. Tape down too ticking paper and use to to transfer wither on paper or direct on the mattress. Hard to follow but watch a video and it will make sense.
The term V-berth is something of a mislabel when applied to the C-310. We still call it the v-berth, but it would more accurately be called the forward berth, since it is essentially a walk-around berth. One of the reasons we bought the C-310 is that a couple can lay down with their heads either forward or aft. Reading lights are installed at the forward end of the berth, which work very well for us.
We have always used standard queen sheets and folded the excess material under the mattress, but a custom fitted sheet would sure be nice!