jings I have even considered putting a bit of velcro to hold the top of the duvet up against the top of the cover, but I think it would be uncomfortable.
kitty The duvet can be the same shape as the bed without being the same size - the overhang is even on all sides if the shape is the same.
I put a rug over the foot of my bed in winter and tuck it in to hold the bottom of the duvet in place, otherwise it either slides further and further down and uncovers my shoulders, or else my feet stick out the end and let in a draught. (sounds like the tramps blanket) In summer the rug is replaced by a folded cotton bedspread.
My spare bed is an oldfashioned one with a footboard, on that one the end of the duvet is tucked down between the footboard and the mattress.