I was an on-again off-again user of the grip on a D80 ...it does loosen up a little over time but was never bad enough to become a problem. Spare doors are possible too, very cheap as I recall from Nikon Parts but $$$ on ebay. If you can get a closer look at the hinge just the once, you will see the line of attack and how the hinge fits. Even if it loosens up a bit it will not come off when closed or fully open and the battery will not fall out 99% of the time.
Not sure if the D80 and D90 doors are interchangable as a spare part, must find out.