Before you break out the duct tape, see about fans! Or plan to veg JUST long enough to gain height, and then strip the first few inches of the stem from the bottom up, so that there's no extra foliage down there to hold in the damp.
Hint: If you want to increase stretch, use your HPS in late veg/early flower, and allow a 15'F difference between night and day. Blue light and steady temps reduce stretch.