+xHave never managed to make a fuel cap that didn't have this issue haha, so dont beat yourself up. Luckily with pre-preg you can be very neat about it, so I find it easiest to cut your plies so that they are only 1 or 2mm bigger than the part and you have very very little overhang material. Obviously this means you have to be very careful placing the plies into the mould so that you don't end up short anywhere, but it should be quite easy. What this will mean is that there is very little material holding on to the flange, which will allow it to slide into place easier.
As a side note - covers should not be so tight that a single overlap of carbon makes them not fit, so it should be perfectly possibly to do it with a slip joint as well. You're only talking about ~0.25mm, and you realistically need a gap for adhesive or whatever you are going to use to hold the cover in place, so it shouldn't matter.
Also, iv tried everything but can't get the curves uniform and smooth ish to fit tidy. I'm having the dremel sand the back of the part. Any advice?