I ordered rail buttons for the first time and was looking how I wanted to mount them. I wanted a flush install so I could remove them if I wanted to. I came up with this bracket that sits on the inside of a 3.0" body tube. I'm sure I overcomplicated things but let me know what yall think!
I did something very similar on my 4" Fiberglass Tube Fin. This required me to offset the exterior rail button a bit so the rail didn't hit the tube fin. 4" is a lot of interior space, and I mounted the rail guides low, so I wasn't worried about interfering with the recovery harness.
I've also done something very similar for smaller diameter rockets, but there I also contour the interior holder. Same thing for camera shroud interior holders.