Try using the Slides Toolbox. Play a rapid chromatic scale over the interval of the desired glissando (fitting it also to the timing). Make sure the note values are long enough to overlap one another and then experiment with the slide interval by using the mod wheel to choose between minor 2nd, major 2nd, and major 3rd slide intevals. Keyswitching chooses between up and downslides. Obviously, use upslides for ascending glissandos and downslides for descending. So, the main two parameters to experiment with are note length (overlaps) and interval size to get the smoothest transitions between slide samples. Some other things to play around with: Scale type (chromatic, whole tone - probably best to use symetrical scales like these to keep the intervals consistent) and level with cc#11 to adjust the dynamic shape of the gliss.