I purchased a set of PBT keys to replace the wearing stock ones on a MJ2. The profile is slightly taller than that of the stock keys. The keys mounted directly on the switches work great, and the 2u-sized keys have only a small amount of rattling or imbalance. I think this is because there is mininal tilt during press of those keys. However, if the spacebar is pressed anywhere other than the direct center, the cap slides off the insert on the opposite side, instead of depressing it. I have tried wrapping the inserts in plastic and teflon, but this does not fix the problem at all.
Are my (costar) inserts just worn out? Was the spacebar poorly molded? I have noticed there are not many options for different plastic inserts, which I would definitely get if they would fit more snugly into the keys.