This is a very old idea and probably covered by scores of 19th C patents. Problems are variable friction (changes in temperature and moisture/dirt/polishing on the friction surfaces) and acceleration (the terminal velocity is reached after a few meters - the top half of the lens get longer exposure than the bottom half). A certain amount of weight in the slide would help reliability. If I make one, I would use a set of strips (very much like a set of Waterhouse stops) with different slot widths rather than fiddling around with adjustments.