I like the idea of saving weight by removing a pack's top lid, but would like something better than just a drawstring closure. Here's a mod I worked out that allows roll-top closure without lid, and easy conversion back to lid + drawstring.
After removing the lid, I replaced the drawcord with two strips of plastic strapping (free from lumber yard). This stiffens the bag top and helps secure the roll-top. Weight of strap is less than drawcord the pack came with.
I attached 3/4" side release buckles to the bag top with velcro tabs. Buckle is sewed to 3/4" webbing, which is then sewn to the hook side of the velcro.
Loop side of velcro is sewn to bag top on long edges. Loop side won't snag fabrics if drawcord is re-installed.
Bag rolled closed and buckle snapped into male end on existing top lid compression strap.
Bag fully closed. Roll-top conversion is 0.4 oz weight penalty versus drawcord closure (removing lid drops ~ 5 oz). Pack is a Jansport Bivouac 49, bought a few years ago on close-out for less than $40. Starting weight was 2# 13 oz. With top lid, framesheet and hydration pocket removed, and straps shortened, weight is now 30 oz. Could probably drop another 2 oz with more surgery.
Next, I plan to apply the same conversion to my Exos 58.