Just as a follow-up. I finally had time to try out the RATech steel bolt catch for M4/M16...
There is only one spot that needs to be modified like the stock bolt catch. Not sure what RATech was thinking, but the bolt catch can barely pivot upwards to catch the bolt if the mod was not completed. The rectangular piece (pic below) simply need to be grinded down so the bolt catch can rotate upwards:
Here is all of them all modded:
Enjoy, hope this helps!