You could do it the way all the cheap zoomies are made - by having a threaded pill hold the O-rings as well as retain the slider in place. You could then either (a) insert the lens last, or (b) use the friction of the o-ring to get the pill threaded down, then finish tightening it from the inside of the tube by turning it counter-clockwise.
For the lens, you could machine a very shallow groove into the inside of the head, heat it and place the lens. It would take almost nothing to retain the lens sufficiently