For my on camera mic I use an old Sony shotgu style mic that came on an older Sony ENG camera and it is by no means a primary mic but is great for nat sound.
I have a few other Sony mics like this and none of them ever fit in the mount without a spacer. They’ve all come with rubber spacers.
Also, any shotgun mic I’ve ever purchased has come with either rubber O-rings or a rubber sleeve to use on the camera mount to keep it in place.
I use these: 2 pack Microphone Spacer Rubber... https://www.amazon.com/dp/B07HCFGSDZ?ref=ppx_pop_mob_ap_share
I cut the rim off the second one and put it inside the first. Works like a charm