I thought it would be a good idea to post the method I use to install gear doors. There have been a lot of posts lately asking for guidance, so I hope this helps.
I don't have an unassembled model to use for this, so I took pictures from a solid model I created. If you use this method your gear doors should come out just about perfect.

Disclaimer: this is how I do it, and it has worked for me. If you don't like it, I don't care, just don't use it. But guess what, this is how it is done on Mach 2 fighters.

I selected a curved surface similar to an F-16, it would work the same on a flat gear door such as an F-15, only that it would be much easier.

Good luck!