What MJ said!! Can you say coil bind? Be careful with this mod....on my old springs the coils bind (touch each other) at 1.4575" or just over 1-7/16" compressed height. I installed new springs and measured the amount they are compressed to be roughly 11/16". This only leaves about 1/8" more compression before binding will occur on my new springs. Once the coils touch it's the same as a solid piece of metal, so all the pressure causing your cover to bend is trying to crush that plastic bushing!
Shimming old sagging springs to restore preload would be ok if you do not bind the coils, but there is not a lot of room to play with here. If the spring has sagged then the coils will be closer together so the distance you can compress it will be less. This will make it difficult if not impossible to shim it enough to get it back to the same force as new and not bind the coils. If the plastic bushing has worn so preload is reduced then just replace the bushing.
Hum, replace the upper bushing with a bearing?
POR
BTW some spring info (all approximate)
Old spring free length = 2.17"
New spring free length = 2.29"
Estimated installed height = 1.6"
Measured spring rate (old spring) = 850 lbs per inch
Estimated installed spring force (new spring) = 587 lbs
Estimated installed spring force (old spring) = 485 lbs