is this the kind that has a lower level that pops up? or a top-only that has metal or wood work on the back and two sides?
if it has only the back and two arms, you would make it in a "T" shape, and bind it off that shape, as they did in colonial times. if it pops up, make a skirt that attaches to the frame beneath the mattress with velcro. the skirt should be made in three parts. the front, the side and the other side. if the skirt has an upright ruffle or gather, it will disguise the next step. the top need only be a little larger than the top of the mattress + the overhang. for instance, if the mattress is 36" across and is 5" high, the quilt should be 36 + 5 = 41" + enough to tuck under the mattress.
let's say that's another 5" for good measure. that gives the total of 36 (top depth) + 5 (height of mattress) + 5 (tuck).
you need a total of 46" for the width and you need to add the same extra 10" for the length, for the same reasons. at the corners, you still have to create a "T", but it will be much shallower, because the quilt won't be hanging to the floor now.