ive done it with 10mm fittings, i would expect the 3/8 to be the same size to be honest, as their dimensions arent particularly critical in their intended use. have u tried to get the fittings on from both ends of the cup? mine wouldnt go on from the open end, but fit snugly when pushed on from the other end, i also scuffed both the inside of the fittings, and the outside of the cup with sand paper and then ca glued them on. perhaps they need a little "encouragement" to go on..?