- having the slack chain over the top keeps them mostly away from the dust and if you cut smaller parts, allows to slide your sheet left/right to have the part centered on the frame
- look up the weight tensioning. Far more consistent than the bungee cords and with milk bottles filled with sand or water fine adjustable. How not to do it is here Poor man weight chain tensioning , there are far better solutions in the Forum.
Kind regards, Gero