Most of the folks I know make the quilt big enough for the final twin bed size, with whatever overhang you will want at that time. For now, you tuck the inside part under the mattress so the quilt stays on the bed. If you don't put borders, it won't be so obvious that you may be letting it hang longer on the front now. After the back side of the bed is removed and you have a true twin, you just center the quilt on the bed.